PREPOSITIONS form a pretty exclusive club. Unlike nouns and verbs, of which there are squillions each, Wikipedia lists over a hundred modern one-word prepositions, a few two-word ("next to") and three-word ("as far as") phrases that act as prepositions, and a few archaic ones ("unto", "betwixt").

A hundred-odd might be more than most people think of when they think of prepositions: you could probably rattle off "on, in, under, around, behind, about..." off the top of your head before getting to a dozen or two and starting to run out of ideas. You might not think of "pace", "versus" or "notwithstanding". Prepositions aren't really a club with a membership committee, of course. In grammar, a word belongs to a class if it has the properties of that class. A preposition heads a prepositional phrase, and usually takes a noun phrase as its complement. And so in "Notwithstanding the rain, it was an excellent night out", "notwithstanding" can only be a preposition. New prepositions can appear: Geoff Pullum spotted one coming into being here.

But it's still true that the class of preopositions is pretty small. And if a month ago you had told me there was a fully established preposition I'd never heard before, I'd have bet money against it. And lost. In the past two weeks, the preposition "modulo" has been used in an e-mail to me (by Philip Resnik) and in a Language Log post (by Mark Liberman). "Modulo" is on Wikipedia's preposition list, and has an OED entry.

If you're not yet modulo-savvy, here are the two contexts:

Resnik: "Modulo some minor differences in the analysis that don't matter for this discussion..."

Liberman: "Modulo this uncertainty, here's my replication of the same ratio from the same source...:

The meaning is not hard to guess. The OED gives it as "(a) With respect to an equivalence defined by (some feature), disregarding differences indicated by (some unimportant feature); (b) taking into account (a particular consideration, aspect, assumption, etc.)."

The origin is mathematical. The OED says "modulo" is "with respect to a modulus", and under "modulus" gives a citation explaining: "If two numbers a and b leave the same remainder when divided by a third number c, they are said to be congruent with respect to the modulus c."

So 10 modulo 3 is 1, and 22 modulo 3 is also 1. We can further say that

10 and 22 are congruent to each other modulo 3.

The final figurative leap is this:

"A is the same as B modulo C",

means A is the same as B except for differences accounted for by C.

Google's n-gram viewer shows "modulo" as much less common than other fancy-pants prepositions like "qua", "amidst" and "notwithstanding". Our example sentences above flummox the Stanford Parser, which fails to analyse "modulo" as a preposition. So if you already knew how to use "modulo", a gold star for you. And if (like me) you thought you knew all the prepositions worth knowing, but didn't know "modulo", just enjoy the surprises that language can constantly throw at you.