# The universal catalogue

The Universal Decimal Classification aims to label all human knowledge, and it’s even more thorough than the Dewey Decimal system.

This is the 800th post on this website, and I wanted to mark the occasion by revisiting a question from last year: just how do you classify all human knowledge?

As I noted in the post on categories of knowledge, any classification system inevitably reinforces the social, cultural, and intellectual biases of its creators. The 1932 edition of the Dewey Decimal System notoriously filed same-sex relationships under “mental derangements” and “abnormal psychology.” Nevertheless, certain institutions require some method of arranging their contents. Imagine visiting a library without one!

Two 19th century Belgian bibliographers heard about the Dewey Decimal System and asked to translate it into French. But rather than slavishly follow Dewey, they added some significant twists. Their system, first published in 1905 and still used today, is known as the Universal Decimal Classification.

(The two bibliographers became quite famous for other reasons. One created a turn-of-the-century manual search engine and the other received a Nobel Peace Prize. But those are stories for another time!)

The problem with Dewey is that it’s hierarchical: headings, subheadings, sub-subheadings, and so on. This makes it difficult (but not impossible) to capture connections between related topics. Say you have a textbook on 19th century religious education in France. Do you file it under religion? Education? The 19th century? France? Textbooks? The Dewey system captures some of these nuances (the decimal points for some topics subdivide it by places and times). But on the whole that’s a messy and imperfect kludge.

The Universal Decimal Classification handles this much better. It begins with ten overarching classes, just like Dewey. But it also introduces auxiliaries, symbols and punctuation that let you combine numbers in any way that fits. A colon indicates a simple connection between two topics, for example. So you could link education (37) and religion (2) together with the simple “37:2.”

Numbers in parentheses indicate a location. Quotation marks indicate a time or time range. An equal sign before a number indicates a language. Numbers beginning with a 0 and in parentheses indicate the medium or form of a source.

Say, for example, you have a PDF of 1970s tourist maps of Grafton County, Maine. Under Dewey, you’d probably put it under tourism, or maps. But the UDC number captures all of its different facets with the verbose and complete classification “338.48(734.211.4)”197″(084.3)(0.034.2PDF).” This breaks down as follows:

338.48 Tourism
(734.211.4) auxiliary number of place: Grafton County [USA, Maine]
“197” time auxiliary number for 1970s
(084.3) auxiliary number of form – map
(0.034.2) auxiliary number of form – carrier – digital file

UDC structure and tables

It’s pretty clever and also fiendishly complicated. The Universal Decimal Classification is in use today in 130 different countries and up to a quarter of a million libraries and document collections worldwide.