Taxonomy is easy. You have a list of concepts, terms and you put *this* under *that*. Often, even if not completely explicit, the reason for putting *this *“under” *that *is intuitive:

Me: Does *Algebra* “go under” [become a Narrower Term of] *Mathematics*?

Me: Yes.

**Blogpost Taxonomy
**Mathematics

-Algebra

This seems obvious; we mean something like: “Is all Algebra Mathematics?” Things can, however, quickly get more complicated:

Me: Is a *Collie* a *Dog*?

Me: Yes (as a *Collie* is a breed of *Dogs*; all *Collies* are *Dogs*).

Me: Is *Lassie* a *Dog*?

Me: Yes (or, was; *Lassie* [the concept of *Lassie*] is still an instance of a *Dog*).

Me: So…*Dead dogs* are also *Dogs*?

Me: Um. Yes.

Me: Huh.

**Blogpost Taxonomy
**Dogs

– Collies

– Dead dogs

– Lassie**

Mathematics

– Algebra

This looks a little strange; those three terms do not look like siblings, or as though they’re in the same “category” of things. It’s clear that they don’t really all have the same [expressed] relation to *Dogs*, so exactly what does it mean that they all have the same Broader Term? By the book* there are three valid circumstances under which a concept (Term B) can “go under” or become a Narrower Term of another concept (Term A):

- Generic: Term B is in the genre of—a subclass or subtype, such as a sub-discipline (
*Algebra*and*Math*) or a breed (*Collie*and*Dogs*)—Term A; - Instantive: Term B is an example of an individual instance (
*Lassie*is a*Dog*) of Term A; or - Partitive: Term B is a part of Term A: like the engine of a car, the trunk of a tree, or…the leg of a dog.

Therefore, the following is a standards-compliant structure:

**Blogpost Taxonomy**

Dogs

– Collies

– Dead dogs

– Dog legs

– Lassie

Mathematics

– Algebra

“Broader Term” is clearly doing several kinds of work here, which are not distinguished from one another; the *Dog* branch of our nascent taxonomy above looks like the taxonomist (in this case: me) is asserting that all four of the Narrower Terms (siblings) shown are, in some sense, “of a kind” when they are clearly not. Ideally, a good vocabulary structure would try to distinguish between these relationships; this seems especially true when vocabularies are being used to power things like machine learning, voice search, and other technologies.

The problem, then, is to clarify the various BT-NT relationships in the *Dog* branch in the emerging **Blogpost Taxonomy**. Still going by the book, we have several options.

**Option 1: Add intervening terms**

We can certainly clean up this branch by interposing some intervening topics; something like:

**Blogpost Taxonomy (***Dogs*** branch only)**

*Dogs*

Dogs

– Canine anatomy

— Dog legs

– Dead dogs

–Lassie***

Dog breeds

– Collies

– – – Lassie***

Famous dogs

– Lassie***

This is effective enough and could be a good solution—particularly if you have lots of other terms that also fit into those second-level categories of the topic *Dogs*. This is common in many taxonomies, and for many purposes I might advocate this approach; taxonomies constructed this way are friendly for tasks like document indexing and retrieval and site navigation.

This is because, for those purposes, it’s not important that *Dogs* is a *topic* and the second-level terms are *subtopics*, while starting at the third level we start to have *things covered by those topics*. The interposition of the subcategories smooths over the jarring dissonance of having *Lassie* and *Dog legs* as siblings; simultaneously, we maintain the integrity of our all-some, or BT-NT, relationships.

It’s still unsatisfactory on a philosophical level—or if you want to use your taxonomy for the basis of some kind of machine-readable application—that the types of Broader-Narrower relationships are conflated into a single relationship type; one we can’t refine.

**Option 2: Facets**

We could add some facets to clarify the types of things in our taxonomy: categories for our categories. These can take many forms; the set of permitted facets in a vocabulary effectively comprise a little authority file of their own.

**Blogpost Taxonomy: Facets**

Breeds of dog

Dead animals****

Instances of dogs

Parts of dogs

**Blogpost Taxonomy (***Dogs*** branch only; Hierarchy View)**

*Dogs*

Dogs

– Collies [Breeds of dog]

– Dead dogs [Dead animals]

– Dog legs [Parts of dogs]]

– Lassie [Famous Dogs]

**Blogpost Taxonomy (***Dogs*** branch only; Facet View)**

*Dogs*

Facet

– Term Broader Term

—————————————————————————–

Breeds of dog

– Collie [BT: Dogs]

Dead animals

– Dead dogs [BT: Dogs]

Famous dogs

– Lassie [BT: Dogs]

Parts of dogs

– Dog legs [BT: Dogs]

Facets are useful and underutilized, I think, as faceting allows a third dimension in relating concepts. Here we’re asserting that not only is *Lassie* a *Dog*, *Lassie* also belongs to the “category” of *Famous dog*. Again, for some vocabulary use cases this architecture is an excellent choice.†

But isn’t this solution merely recasting the interpolation of terms in Option 1 (without the benefit of tidying up the hierarchy)? The problem—casting terms in relation to one another without a way to really define those relationships—remains. We have plenty of ways to describe and categorize terms; we want the same control over the *relationships* between them.

**Option 3: Subtypes of Broader-Narrower Term Relationships**

A seldom-used design (also outlined in Z39.19) takes advantage of the three valid BT-NT relationships mentioned above (Generic, Instantive, Partitive) and provides each with its own designation:

- Broader Term Generic (BTG) – Narrower Term Generic (NTG)
- Broader Term Instantive (BTI) – Narrower Term Instantive (NTI)
- Broader Term Partitive (BTP) – Narrower Term Partitive (NTP)

…essentially allowing three types of Broader (and Narrower terms) to help clarify exactly the ambiguous relationship type we’ve been poking at, like so:

**Blogpost Taxonomy [specific BT-NT version]**

Dogs

– NTG Collies

– NTG Dead dogs

– NTI Lassie

– NTP Dog legs

Mathematics

– NTG Algebra

Combining this result with Option 1’s interpolations, a somewhat satisfactory result can be obtained:

**Blogpost Taxonomy (***Dogs*** branch only)**

*Dogs*

Dogs

– NTG Canine anatomy

– – NTG Dog legs

– NTG Dead dogs

– – NTI Lassie***

– NTG Dog breeds

– – NTG Collies

– – – NTI Lassie***

NTG Famous dogs

– NTI Lassie***

This is a little cumbersome, but it helps distinguish between the various NT relationships—to a point. (Note also, interestingly, that we’ve lost the NTP-relationship for *Dog legs* by changing its parent.)

The use of this (three-kinds-of-BT/NT-relationships) architecture is not widespread; it’s hard for machines to know what to do with this structure, and it’s really a half-measure towards what we’ve been interrogating all along: to *precisely specify and* *name the relationships* between the concepts in our vocabulary. I theorize that this architecture was essentially superseded by the spread of graphs.

**Option 4: From Taxonomy to Graphs**

If we conceive of our vocabulary as a graph (which it is; technically and specifically a directed acyclic graph:‡

…with terms as the nodes and their relationships as the edges, we can recast our problem: *we want to be able to name the edges*.

And so we come to the reason we’re hearing about the “rise of knowledge graphs” in recent books and articles and at conferences: it’s solving a problem in vocabulary architecture that’s becoming increasingly important as the foundation of AI and other technologies (not to mention search).

Essentially, this approach uses *ontological* *RDF structures* to model controlled vocabularies (terms as well as their relationships; nodes as well as edges) in a way that is *machine-readable* and *interoperable*.

Building graphs is, admittedly, more difficult than building taxonomies; modeling vocabularies using this richer but more complex architecture requires care and precision, which equals time and effort.

But: *we get to name the edges*.

More on this in Part 2 of this post.

**Notes**

* ANSI/NISO Z39.19 (2005; rev. 2010)

** Obviously Lassie could also be an NT of *Dead dogs* and *Collies*.*** Clearly, I could not resist the polyhierarchy.

**** This taxonomy is shaping up to be rather more morbid than I had planned.

† The Getty vocabularies, e.g., famously, make extensive and complex use of facets.

‡ I am, after all, a taxonomist.