What's a thesaurus?

We recently released a new version of Atramhasis, our web-based thesaurus editor. But what actually is a thesaurus, and why would a heritage organisation want one?

Our colleagues at Historic England define a thesaurus as a structured wordlist used to standardize terminology, used to assist in indexing and retrieving information within databases that make use of the same terminology. Structuring related terms like this opens up a few useful things.

First, it lets you express equivalence. Rather than saying two terms are "synonyms," we say both are verbal expressions of the same underlying concept — so instead of stating Jugendstil equals Art Nouveau, we say there's a single concept for that artistic style, labelled with both terms. Subtle difference, but it changes how you interact with the thesaurus.

Second, there are hierarchical relations: one term broader than another, the other narrower. A cathedral is a church, a church is a religious building, so a cathedral is a religious building too. Not all hierarchical relations are strictly "is a" — "is part of" and "is an instance of" exist as well, and they complicate matters. See On the composition of ISO 25964 hierarchical relations (BTG, BTP, BTI) for how they interact.

Third, the associative relationship — the "softest" one, just stating two terms or concepts are related. Coal mine relates to miner's houses because in Flanders, every coal mine had its own little town where the miners lived.

Thesauri also let you attach a scope note: a written definition of a concept and how it should be used. This turns out to matter a lot in practice — asking experts to define concepts they assumed were self-evident has caused more than one heated discussion. Without a definition, everyone uses a concept however they like, ignoring that others might disagree.

So thesauri help us reach consensus on what concepts mean and share that with the outside world (which is why we publish them as open data). But we also have a very practical reason to build and maintain them: indexing. When someone searches for fountains in our inventory system, the thesaurus is consulted. If fountains isn't a known term, nothing gets indexed under it and the search stops there. If it is, the system also checks for narrower terms — in this case water tricks is a defined narrower type of fountain — so a search for fountains returns records indexed as either fountains or water tricks.

Those water tricks also show off another handy feature. Look at the record for water tricks and you'll see we call them bedriegertjes in Dutch but never got around to an English label. Luckily, we've linked our bedriegertjes concept to the same concept in the Art & Architecture Thesaurus, which tells us the English terms are water tricks or joke fountains — and even gives us the German and Spanish equivalents. That opens up a lot of possible interactions with other systems.

In the end, thesauri are a means, not an end — our job is managing heritage, not building thesauri. But they help enormously in defining what that heritage is, searching through it, and linking it from our local level to the transnational one. If you want more detail on how we built ours, including some do's and don'ts, the original post pointed to a bibliography entry (Mortier et al., 2017) that we weren't able to carry over exactly.