Wednesday, August 31, 2022

SKOS-XL for Taxonomies

I recently posted about SKOS (Simple Knowledge Organization System). If you have read anything about SKOS, then you might have come across SKOS-XL (SKOS eXtension for Labels) and wondered what that is. The World Wide Web Consortium (W3C) released its recommendations for SKOS and SKOS-XL at the same time in 2009 but chose to make them separate recommendations. One way to see it is that, by separating out SKOS-XL, SKOS is indeed truly “simple.” In the detailed SKOS reference, SKOS-XL is an appendix.


Extending labels to become resources

“Things, not strings” is a tagline for semantic models, such as SKOS, which emphasize concepts in taxonomies and other knowledge organization systems and not terms or words. Of course, strings of text exist, and when associated with concepts they are called “labels.”  The distinction between a label and the concept that the label describes may seem indistinguishable or perhaps just philosophical. The main difference is that concepts are unique within a taxonomy, but labels are not. A concept may have multiple labels (synonyms or names in different languages), and the same label might apply to different concepts (homographs).

SKOS specifies preferred labels, alternative labels, and hidden labels as options for concepts. Hidden labels can be considered as a type of alternative label that should never be displayed. Alternative labels may display, depending on the front-end application. Preferred labels are what are displayed, especially in hierarchies and facets. 

Concepts, as things, have properties or characteristics. Labels do not. But sometimes there are reasons to assign properties to labels, such as to indicate the purpose or use of different labels. In this sense, you would want to turn a string into a thing. More correctly, a thing is called a resource, as described by the Resource Description Framework (RDF) the model upon which SKOS is based. This is what SKOS-XL supports: converting labels to resources. It does this by adding three more elements not found in SKOS: label, label relation, and literal form. It is the label relation in particular that enable the extension to establish a link between label. Further details are in the W3C's SKOS-XL recommendation, which I am not going to repeat here.

Use for SKOS-XL

A typical use case for SKOS-XL to assign properties to labels is if you want to have different labels for different user groups, such as a medical taxonomy for shared medial content to be accessed by both medical professionals and lay people. Medical professionals may prefer a concept labeled Neoplasms, while lay people could call it Cancer. Different user groups could be based in different regions. Although different ISO-code based language labels can be used to distinguish regions in addition to language (such as en-US and en-GB), you may not want to duplicate the vast majority of preferred labels and merely distinguish the few that are actually different.

While SKOS permits multiple alternative labels, aside from hidden labels, there is no way to distinguish their types or purposes in SKOS. You may want to alternative labels support search in one front-end application and not another. You may want to designate official acronyms as distinct from other alternative labels. You may even want to distinguish between different kinds of hidden labels, such as those that should be hidden because they might be pejorative or offensive, and those that you wish to hide only from a type-ahead display because they are near duplicates of other alternative labels and too many alternative labels would clutter up the display.  Finally, there may be alternative labels used by only certain users or in certain regions.

SKOS-XL lets you assign properties or attributes to labels. Assigning the purpose or use of the label is only one possibility, although it is the most common use of SKOS-XL. You may wish to manage more administrative metadata about labels, such as the source or origin of different labels. 

Implementing SKOS-XL

The principle of SKOS-XL is not complex, but implementation can be more challenging, and if you are building taxonomies with the SKOS-XL capability, you would want to use taxonomy management software that supports SKOS-XL, such as PoolParty. Taxonomy management software products are quite consistent when it comes to their user interface for supporting the editing of basic SKOS taxonomies, but they are not the same for creating and editing SKOS-XL labels, which is a less common function. 

Having properties, such as types, for terms is not new, but required some more innovation in the SKOS model of things (concepts), not strings (terms). It was common for non-SKOS taxonomy/thesaurus management software, which treated different terms with the same meaning as equivalence relationships, to support the customization of relationships, including the equivalence relationship. SKOS-XL ensures that this earlier feature is supported in the current standard, in machine-readable format.

For SKOS-XL to be more widely used and maybe even more elegantly supported requires a great sharing of use cases. I hope the taxonomist community will share their experiences with SKOS-XL, so we can talk about practices and recommendations and not just theory.

Further information:

Sunday, July 31, 2022

Taxonomy Challenges Discussed at SLA Conference

When it comes to conferences dealing with the subject of taxonomy creation, implementation, and maintenance, without a doubt Taxonomy Boot Camp and Taxonomy Boot Camp London are by far the best conferences for their content, speakers, and networking opportunities. However, there are other conferences that have sessions on taxonomies. 

The annual conference of the Special Libraries Association (SLA) usually has multiple taxonomy-related sessions. This year, July 31 - August 2 in Charlotte, NC, the first in-person conference in three years, was no exception.

Thanks to the volunteer programming efforts of SLA’s Taxonomy Community (one of over 20 specialized topic groups, formerly called “Divisions"), the annual conference is able to include multiple taxonomy sessions, some of which bring together multiple speakers, either co-presenting a single talk or coming together. Even sessions not organized by the Taxonomy Community may include taxonomy topics, such as those dealing with knowledge management, information architecture, or research that uses a taxonomy. A Taxonomy Community networking event is also regularly part of the SLA conference.

This year’s conference is hybrid, so some of the taxonomy sessions are in-person, and some are pre-recorded and available on-demand.  Live-streaming was also done for keynotes and some sessions. The following are the in-person taxonomy sessions at the SLA 2022 conference:

  • “The Role of DEI in Taxonomy Development, Maintenance, Search, and Retrieval,” presented by Marisa Hughes. (This presentation on a popular topic was additionally live-streamed and pre-recorded for on-demand viewing.)
  • “Current Challenges and Advanced Taxonomy Topics” panel comprising Marisa Hughes, Heather Kotula, John Bertland, and myself.
  • “Research Sources and Methodologies for Taxonomy Development,” jointly presented by Marisa Hughes and myself.
The following are pre-recorded, on-demand only taxonomy sessions:
  • “There ain’t no Sanity Clause: Taxonomy and Data Analysis” presented by Michele Lamorte
  • “Metadata Governance” presented by John Horodyski

Conference session on diversity, equity, and inclusion in taxonomies

Diversity, Equity & Inclusion (DEI) is a growing area of interest in information management/sharing and content creation.  Marisa Hughes, the taxonomist who edits the APA Thesaurus of Psychology Index Terms explained the challenges of revising the thesaurus terms to reflect DEI, for which she gave the following definitions:

  • Diversity: “The vast range of differences among individuals and groups.”
  • Equity: “The contain of being fair and impartial”
  • Inclusion: “Welcoming and respecting diverse individuals and Groups. Diversity in practice. 

She has been reviewing thousands of terms for accuracy, currency, inclusivity, avoidance of bias, stereotypes, or discrimination. Areas that this DEI review has focused on are:

  • Racial, ethnic, and cultural identity
  • Gender diversity and sexual orientation
  • Age, disability status, and socioeconomic class bias

In the area of disability status, for example, the term should focus on the disability and not the person. Thus, “Hearing impaired” is changed to “Person with hearing loss”; and “Mentally ill” is changed to “Individual with a mental illness.”

Marisa Hughes presenting “The Role of DEI in Taxonomy

Additional challenges include taking the hierarchical relationships, term usage, and change management. If users can see hierarchical relationships, even if not the full hierarchy, these relationships need to be appropriate. For example, certain personal conditions and behaviors should not be narrower to the term “Disorders.” Term frequency of usage (also called “literary warrant”) is important, but the larger goal is to have respectful terms. Change management involves care that the term changes to not impact search and retrieval. Marissa oversees the large job of reindexing content with new terms, and adding change notes or history notes to changes terms. 

Conference panel on current taxonomy challenges

In this session, the four panelists each gave brief opening talks, then were asked questions by the moderator, Judith Theodori, and then it was opened up for general Q&A and discussion with the audience.

I presented on the themes of challenges which came from 138 taxonomist survey responses to the question "What are the pain points or challenges in your taxonomy work?" The leading trends in the responses were:

  1. Achieving stakeholder understanding and buy-in
  2. Competing interests, expectations, and requests
  3. Organizational challenges
  4. Tools and technology inadequacies or not integrated

John Bertland, Digital Librarian and Content Specialist at the Presidio Trust spoke of the taxonomy challenges in his organization including governance at the time organizational change and funding. A specific challenge is expanding and adapting a taxonomy that was originally just for digital asset management to include the content of the intranet.

Current Taxonomy Challenges” panelists Marisa Hughes, 
John Bertland, Heather Kotula, and Heather Hedden

Marisa Hughes, Taxonomist at the American Psychological Association, related the challenge of having to quickly come up with all the COVID related taxonomy in time for the usual thesaurus update scheduled in April 2020. This involved a lot of research on literature that was still rather lacking on the subject. 

Another challenging project was to determine the role of historical data in the vocabulary of 3500 terms for the period of 1967 to 1973, which involved removing offensive terms. It was a judgement call of whether to continue to use a potentially offensive term as a non preferred term (alternative label) or not.  Heather Kotula, VP, Marketing and Communications of Access Innovations, Inc., the fourth panelist, also discussed the same subject of excluding pejorative terms, referred to “semantic censorship.” In the end it was concluded that often pejorative terms are actually not that much in use in the documents being tagged. 

Sunday, June 26, 2022

SKOS Taxonomies

Over the 26 years that I have been involved in controlled vocabularies, thesauri, and taxonomies, the biggest change I have seen in the field is the adoption of SKOS (Simple Knowledge Organization System) as a schema model and standard.

If you are creating taxonomies exclusively within a single system (such as the SharePoint Term Store or controlled tags or categories of a content management system, documentation management system, DAM, etc.), then you probably have not paid much attention to SKOS. It’s true that taxonomies created within and used within a single system, do not have to follow an external standard. But that is not the trend of information management and technology anymore. Connectivity, interoperability, data sharing and reuse, data-centric architecture, vendor-neutral formats, linked data and linked open data, breaking down data silos, enterprise-wide knowledge, and enterprise knowledge graphs have become the preferred trends and directions.

Different Kinds of Standard

With respect to standards, there exist two basic kinds: (1) standards for design, functionality, and a consistent user experience, and (2) standards for compatibility, interoperability, and machine-readability. For this reason, there are two separate sets of standards for taxonomies and other knowledge organization systems. Another way to think of it is that there are standards for each the front end (user interface and experience) and the back end (computer-readable code) of taxonomies, and they are somewhat independent yet still compatible with each other.

For taxonomies and thesauri, more has been written about the front-end design and best practice standards than the back-end interoperability standards. This is for several reasons. The design and best practices standards (ANSI/NISO Z39.19 and ISO 25964 and its predecessors ISO 2788 and ISO 5964), have been around longer. They are lengthier and more detailed than interoperability standards, and they apply to taxonomies and thesauri regardless of their digital or nondigital format. So, this article will focus instead on the back-end, interoperability standard, which is SKOS.

SKOS logo
SKOS Background

SKOS is a recommendation for "a common data model for sharing and linking knowledge organization systems via the Semantic Web". These knowledge organization systems include thesauri (as defined by the ANSI/NISO and ISO thesaurus standards), taxonomies, classification schemes, subject heading systems, and other controlled vocabularies. SKOS is based on RDF (Resource Description Framework), a World Wide Web Consortium (W3C) standard for description and exchange of graph data. RDF specifies that all statements consist of subject-predicate-object triples, and all resources have URIs (uniform resource identifiers).

The development of SKOS aimed to  build upon RDF to provide a recommended schema for thesauri.  SKOS development was first undertaken as the Semantic Web Advanced Development for Europe (SWAD-Europe) project before being adopted and supported by the W3C in 2004. The W3C formally released the SKOS recommendation in 2009.

Meanwhile, the W3C had been working on other recommendations for web-based ontologies, including RDF Schema (RDFS) and Web Ontology Language(OWL). SKOS is compatible with RDFS and OWL, and elements from the different models can be combined. Furthermore, SKOS can even be considered as a very generic upper ontology itself, and the W3C documentation describes SKOS in terms of OWL and RDFS expressions.

The main types of elements of SKOS are concepts, lexical labels, documentation properties (notes), semantic relationships, mapping properties, and concept collections. (Concepts, concept schemes, and collections are ontology classes, and the others are ontology properties.) In their machine-readable form, the SKOS elements are concatenated with no spaces, such as preLabel, scopeNote, and exactMatch.

SKOS Concepts, Labels, and Notes

SKOS is concept-centric. Making a distinction between concepts and labels is the biggest departure from traditional thesaurus standards and past controlled vocabulary practice. A concept is an idea of something, and a label is a name for that idea. Thus, a concept may have multiple labels. For the organization of a vocabulary, especially as a hierarchy, one of the various labels needs to be designated as the preferred displayed label. The others are alternative labels and its sub-type, hidden label, which may be used to designate that the label should not display to end-users. Labels for the same concept may exist in multiple languages, but there may be only one preferred label per language.

Notation is intended for use as an appending part of a label, such as an alpha-numeric code, which is commonly used in classification schemes.

Documentation comprises various types of notes, including scope note, editorial note, change note, and history note. Definition and example are additional documentation types. Scope notes are commonly used in thesauri to clarify the usage of a concept in tagging/indexing for the specific context of controlled vocabulary and its set of content. They serve an important role for manual tagging. Other note types may be utilized for administration and management of the controlled vocabulary. Definitions may be entered for more technical controlled vocabularies or when the controlled vocabulary also serve the function of a glossary.

SKOS Concept Schemes and Collections

What constitutes an individual "taxonomy," "thesaurus" or other controlled vocabulary? This may not be very clear. SKOS introduces the formal organizing unit called a concept scheme, as a “collection of concepts.” A concept scheme is a single controlled vocabulary, thesaurus, hierarchical taxonomy, facet within a faceted taxonomy, or metadata property within a larger metadata schema.

There are some advanced, lesser used features of SKOS, including in scheme, which allows you to control whether a concept is in a concept scheme regardless of whether it’s within the concept scheme’s hierarchy (which is otherwise the default). There is also a special designation of top concept for the top concepts of a concept scheme, a designation which could be utilized for a front-end display implementation.

Collections are an additional optional way to designate a grouping of concepts for a purpose, such as the taxonomy concepts to be used in only specified implementations or those of subject categories for subject matter expert review. Furthermore, concepts can be ordered within collections.

SKOS Relations and Mapping Properties

SKOS includes what are called semantic relations, although this name could cause confusion, since they are the basic thesaurus relationships (broader, narrower, and related), not customizable semantic relations characteristic of ontologies. These thesaural-type relationships are used between concepts within the same concept scheme. In addition, SKOS specifies broader transitive and narrower transitive, meaning the inheritance of the relationship to additional levels of the hierarchy. This is usually assumed to be the case by default, and thus these specifically transitive relations are rarely implemented, but if there are reasons not to inherit and extend the logical hierarchy by default, then the transitive relations may be used. (I have not come across a use case, though.)

Since SKOS specifies concept schemes, SKOS also specifies an additional set of relation types called mapping properties that are to be used between concepts in different concept schemes or different taxonomies.  These comprise exact match, close match, narrower match, broader match, and related match. Exact match and close match are used to map existing taxonomies together, often so that one is used in the tagging and the other is used in the retrieval. The other mapping relations may be used to extend one taxonomy with another while still maintaining a distinction between the two.

Following is a table of SKOS elements by type (class or property) with the concatenated machine-readable forms.

Implementation of SKOS

Most commercial and open-source taxonomy/thesaurus management software now supports SKOS. There are also simple free tools called SKOS editors. SKOS elements are presented in their full human readable names (such as Preferred Label, instead of prefLabel), so it is intuitive to understand. Thus, taxonomists don’t have to worry about SKOS, but should at least be familiar with its principles. Familiarity with SKOS makes it easier to switch from using one software package to another. Software may vary, however, in how well they support some of the less common features, such as in scheme, collections, and broader/narrower transitive.

Taxonomy/thesaurus management software often has the additional administrative grouping of related concept schemes for the same implementation into what may be called a “project” or “knowledge model.” SKOS mapping relations tend to be used more often across concept schemes that are managed in different projects, rather than within the same project. Within the same project, concept schemes tend to represent facets (which have no relations between them) or ontology classes (which have customized semantic relations between them).

Since all elements of SKOS are standard machine-readable, you can leverage any element with rules for usage, such as for how tagging should be done and how concepts and relationships are displayed. Custom applications of SKOS vocabularies are thus common.

If you want to dive into all the details of SKOS, consult these resources from the W3C:

SKOS is intended to be flexible, and it is more suggestive than restrictive. Thus, a SKOS-based taxonomy or thesaurus could still be poorly designed, and that’s why the other standards for best practices, ANSI/NISO Z39.19 and ISO 25964 are also important.

Tuesday, May 31, 2022

A Taxonomist Community

Taxonomists and others whose work involves taxonomies have not been a unified professional community. Taxonomy development work is interdisciplinary, spanning different specializations, and different organizational functions, including the following:

  • Information services taxonomies and thesauri, developed by those with a background in library/information science, thesauri, and cataloging, and possibly indexing
  • Product/ecommerce taxonomies, that may be developed by those with varied backgrounds but experience in retail and product information management
  • Digital asset management taxonomies and metadata, developed by digital asset managers and others, who might have a background in image and media curation and management
  • Website taxonomies developed by information architects with a focus on the user experience
  • Enterprise taxonomies developed by those who are primarily knowledge managers but have also learned about taxonomies
  • Taxonomies for auto-categorization of large volumes of text, developed by those with expertise in natural language processing, machine learning, and other text analytics technologies
  • Taxonomies, as controlled vocabularies, to support metadata and master data management, developed by metadata architects, data managers, and possibly data scientists
  • Taxonomies in support of knowledge graphs, integrated with ontologies, developed by ontologists and other experts in semantic technologies

Thus, people who work with taxonomies, accidental taxonomists and others, associate themselves with different professions and belong to different groups or professional organizations. These include 

For information architects, the Information Architecture Institute dissolved in 2019 after 17 years, and until now, information architects have temporarily been gathering on Discord servers associated with the virtual IAConference and the World IA Day conference, but these have been relatively inactive at other times of the year. 

Discussion Groups

Taxonomists are thus dispersed among these groups and more. It does not make sense to create a new professional membership association for taxonomists, especially at this time when traditional professional membership associations are experiencing declining membership.

Thus, online discussion groups that do not require a paid professional association membership are a better option. The first taxonomy group, Taxonomy Community of Practice was started as a Yahoo group in 2004. It has become quite popular with over 1000 members posting questions and suggestions about taxonomies. However, Yahoo groups declined, and LinkedIn groups grew, so this group was migrated over to a LinkedIn group, later renamed Taxonomy and Ontology Community of Practice. The problem is that this group, as most LinkedIn groups, is less of a community of practice and more an announcement forum.  People are reluctant to post basic questions, as it might indicate that they are not sufficiently knowledgeable. Another former Yahoo group which migrated to is Controlled Vocabulary, which is focused on activities of metadata and controlled vocabulary development and tagging of digital assets, mostly images.

Communities Discussed at Conferences

The need for a community of practitioners, whether taxonomists, or related specialties, is something that has been raised at conferences. 

At the most recent  Information Architecture Conference (IAC), in April 2022, the co-presidents for World IA Day, Grace Lau and Andrea Rosenbusch, gave a talk “(Re)Architecting a Community discussing their hopes and plans to transform World IA Day from merely a single day annual event to community. 

At the most recent Knowledge Graph Conference (KGC), Katariina Kari led a  brainstorming workshop “Building Ontologies and Knowledge Graphs,” as  “a working group for publicly sharing best practices, stories, and the particularities of our craft of building ontologies and knowledge graphs,” seeking a “soundboard for ideas” that others could participate in. 

The upcoming SLA conference will have a live panel session “Communities of Practice: Where Everybody Knows Your Name” on August 2, 2022, in which I will be one of the panel speakers.

A new Taxonomist Community: Taxonomy Talk

Out of conversations and research conducted by Grace Lau in leading up to her IAC talk on an information architecture community, Grace and I discussed in January the idea of an additionally dedicated taxonomist community. I then invited another taxonomist, Bob Kasenchek, to come up with ideas, including what to use for a free platform. Slack, as used by KGC, was dismissed, since the free version has limited data storage, and old messages get deleted. So, we decided to adopt Discord, as it has been used by the virtual IAC in 2021 and 2022. Taxonomy Talk was launched on April 12, and quickly gained sufficient members that they could contribute ideas and be polled for a name. On May 1 it was named Taxonomy Talk. A charter and mission are still in the works. There are several moderators, including Grace, Bob, and myself. 

As of this writing Taxonomy Talk has just over 300 members. It has a number of dedicated subject “channels,” some of which are:

  • New-to-taxonomy
  • Looking-for-help
  • Conferences-events
  • Jobs-and-opportunities
  • Tools-for-thought
  • Learning-resources
  • Reference-resources
  • Best-practice
  • Ontologies
  • Standards
  • Vocabularies

New channels are created as requested, and we might decide to retire or merge low-use ones.

Discord supports features such as direct one-on-one chats and one-one or group video meetings. There are still features I have yet to learn. 

So, if you are not yet in the Taxonomy Talk Community and want to join:
(Please use your real name to promote networking. Some existing Discord users are continuing to use their Discord nicknames.)

Saturday, April 30, 2022

Polyhierarchy in Taxonomies

A defining characteristic of taxonomies is that terms/concepts are arranged in broader-narrower hierarchies, which may resemble tree structures. A limited number of top concepts each have narrower concepts, which in turn may have narrower concepts, etc., and the narrowest concepts at the bottom of the hierarchy are sometimes referred to as leaf nodes, as “leaf” extends the metaphor of “tree.” The tree model has its limits, though, because taxonomies may also have occasional cases of “polyhierarchy,” whereby a concept may have two or more broader concepts.


People who are new to taxonomies, however, might not consider polyhierarchies, because they tend to think of taxonomies as classification systems. Hierarchical information taxonomies have their origin in classification systems, such as the Linnean taxonomy of organisms, library classification systems, and industry classification systems. Classification systems, however, do not allow polyhierarchy within the system. Originally, classification systems were for physical things, such as books, which can belong in only one place, so there could be no polyhierarchy. Standard classification systems, such as industry classification systems, were developed by governmental, international, or nongovernmental organizations with a primary purpose of gathering and organizing statistical data about classes, and thus polyhierarchy is not permitted, as it would lead to double-counting of members of a class.


The primary purpose of hierarchy in a taxonomy is to provide guided browsing of topics to end-users, who may start out looking at broad categories and then drill down to find the narrowest concept of interest. Thus, polyhierarchy serves the same purpose. The idea is that different people will start at different points at the top of the hierarchy to arrive at the same concept of interest, which is tagged to the same content set. A polyhierarchy should be implemented if the concept’s relationship is correctly and inherently hierarchical in both of its cases. An example of a polyhierarchy is Educational software, which has both Software and Educational products as broader concepts. Educational software is a kind of software, fully included within Software, and Educational software is a kind of educational product, fully included within Educational products.



Taxonomy standards and polyhierarchy issues


Taxonomy/thesaurus standards (ANSI/NISO Z39.19 and ISO 25964) describe three kinds of hierarchical relationships--generic-specific, generic-instance, and whole-part,--and polyhierarchy may exist within any of these types. Polyhierarchy that combines different hierarchical types, however, can be problematic, so it is best to avoid mixing hierarchical relationship types. For example, the following polyhierarchy mixes different types:


Washington, DC

Broader: United States (whole-part)

Broader: Capital cities (generic-instance)


The reason to avoid creating a mixed type polyhierarchyis simply that the browsable hierarchy user experience can get compromised and potentially confusing. Extensive hierarchies with large numbers of narrower concept relationships would result. A hierarchical taxonomy tree should be designed with a dominant hierarchy design. An exception is a thesaurus, which is not designed so much for top-down browsing but for browsing from term to term. Mixing hierarchical types within a thesaurus is thus acceptable.


It is also recommended to avoid creating hierarchical relationships across different facets in a faceted taxonomy. This is because facets are designed to be mutually exclusively, so that concepts from multiple facets can be used in combination to limit/filter/refine a search. As such, facets are designed to be distinct aspects. There could be an occasional exception of polyhierarchy, though, but more than 2-3 polyhierarchies across an entire faceted taxonomy should be a cause for review.


With the wider adoption of the SKOS (Simple Knowledge OrganizationSystem) model for taxonomies and in taxonomy management systems, taxonomies are more commonly organized into concept schemes. A concept scheme can be represented as a facet in a faceted taxonomy, but it is not limited to use as a facet. Utilizing concept schemes, it makes sense to have separate concept schemes with different hierarchical types, some for generic-specific (for type, categories, topics), one or more for whole-part (geography, organizational structures), and some containing lists of instances (named entities). In this model, Washington, DC, would be narrower only to the United States in the whole-part hierarchical concept scheme for geographic places. It could also be linked to Capital cities, which is in a different concept scheme for place types, with a different kind of relationship (“related” or perhaps a semantic relationship from an ontology).


Although SKOS permits hierarchical relationships across different concept schemes, it is best practice not to do this but rather to create hierarchical relationships and polyhierarchies confined within a concept scheme, just as it is recommended not to have polyhierarchy across facets.


Additional polyhierarchy considerations

Polyhierarchy concerns concepts in the taxonomy, and it is not about objects, items, or assets that get tagged with taxonomy concepts, such as an individual publication, document, image, product record, etc. Each of these may get tagged with multiple taxonomy concepts, and as such may have multiple “classifications” and thus can appear as if they are in a polyhierarchy, if a frontend application displays tagged items as if they are leaf nodes in a taxonomy.

A polyhierarchy usually involves only two broader concepts, not more. Having more than two broader concepts is extremely rare. If you find yourself creating polyhierarchies of three or more multiple times in a taxonomy, check to make sure you are not doing something wrong with the hierarchy design.

Some content management systems, which have built-in taxonomy management and tagging features, do not support polyhierarchy. The best known is SharePoint with taxonomies managed in its Term Store feature. Taxonomy terms may be “reused” across Term Sets, but they are not permitted within a Term Set, where it is most suitable. See my past post, Polyhierarchy in the SharePoint Term Store, for more details