I came across this sentence in the American Heritage Dictionary, but still do not understand it.

This proposal is the best so far, modulo the fact that parts of it need modification.

The definition of modulo provided is correcting or adjusting for something, as by leaving something out of account.

Please elaborate on the meaning of modulo the fact. Does it mean the same as the following?

This proposal is the best so far, but note what parts of it still need to be modified.

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    J.R.: I've never come across this usage, and it's not in any of my printed dictionaries. On such matters I wouldn't trust Wiktionary either - it's obviously a new (ignorant, imho) usage loosely based on the mathematical sense (as familiar to programmers the world over, today). I can transparently read the intended meaning as "except", but I'd make the mental side-note "writer is a wally". Jun 5, 2012 at 11:23
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    The OED offers 'In extended use. (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.).' There are supporting citations from 1953 to 1992. But I too have never come accross it, and can't think why any writers would want to use it, other than to show that they know it. Jun 5, 2012 at 12:01
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    I immediately took it to mean "aside from", based on a math and computer background I guess, and unlike Fumble and Barrie, I like it!
    – JeffSahol
    Jun 5, 2012 at 12:31
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    @Barrie I come across it fairly often in my neck of the woods. (And by "my neck of the woods" I mean "the Internet".) So often, in fact, that I am very surprised to learn there's anyone not familiar with it. Oh, and it should be noted that the OP's quote is taken straight out of the American Heritage Dictionary definition of modulo.
    – RegDwigнt
    Jun 5, 2012 at 12:52

7 Answers 7


The pattern

X modulo Y

is an informal but common parlance in technical, especially mathematically, oriented talk. It is used to mean informally 'X, ignoring Y'. For example,

"The rocket design was flawless, modulo the toxic waste produced by its fuel."

The meaning is inspired by, but not perfectly corresponding to, the arithmetic modulo function (for example, clock-time addition) which when suitably abstracted involves 'collapsing' all items of a set into the special items of the set, so that the full set does not need to be dealt with (this is where the associated meaning of 'ignoring' comes from).

In your interpretation "note what parts of it still need to be modified", the 'modified' part is irrelevant. 'Modulo' is pragmatically "I'm telling you about the most important part (the X), but remarking on the existence of some part that might be important for other reasons but under the current context we want to ignore (the Y)".


I only see this usage from academics with a background that includes England. It is used to mean the opposite of except - some part of the set is included, not excluded, and you're saying that you're including it even though some people might not. It isn't a substitute for but or except, because those would be about excluding something from the set. It might be closer to even though.

In math, modulo is the remainder after dividing, so 5 mod 2 is 1. In words, it's something like even after accounting for. However, it has been heard by generations of people who aren't sure what it means, don't want to ask, and feel that smart people use it. Those people tend to use it as except or but, meaning that you probably can't be entirely sure any more what someone means when they use it.

  • I started to make a comment, but it seemed worthy of a full answer. However, my answer is just extrapolation, not knowledge of actual use like yours, so I'm voting this answer up.
    – T.E.D.
    Jun 5, 2012 at 12:26

The OED3 has mod as a preposition dating from 1854, and modulo as a preposition dating from 1887 — but those are the more purely mathematical senses, not the extended senses that seem to crop up in the 1950s.

However, programmers and perhaps others regularly use mod or modulo in its extended sense to mean “save/except for”, or “without”, or “minus”. It’s to exclude something. This isn’t a mathematical use, although it may be a form of shop jargon.

Again, it is by no means uncommon in programmer circles, although I don’t know that I’ve myself used it in formal writing.


Modulo is useful in computers for getting a number that can be no bigger than the modulus. So if it were me, I'd take "modulo" in this sentence to mean "limited by".


The example that The American Heritage Dictionary uses for modulo, "This proposal is the best so far, modulo the fact that parts of it need modification," is a confusing and poor usage of the phrase. Unless a reader has a strong analytical mind and understanding of the phrase, they would likely interpret the sentence to mean that the proposal is the best as-is, and it would be even better if it was modified.

On the other hand if the sentence is correctly interpreted, the sentence is meaningless, for a couple reasons. First, a “fact” does not change the quality of the proposal, and so has no impact upon its ranking (i.e. whether it is “best” or not). In general the phrase “modulo the fact” is a poor usage of the word modulo. Second, even if the reader imagines modifying the proposal to be the best, the sentence is still meaningless, because every proposal could be modified to be the best.

Mitch’s example is far better:

"The rocket design was flawless, modulo the toxic waste produced by its fuel."

The best replacement word that I can come up with is “discounting” or "not accounting", so

"This proposal is the best so far, discounting that parts of it need modification."

(Still meaningless for the second reason stated above.)

Or, in Mitch’s example,

"The rocket design was flawless, discounting the toxic waste produced by its fuel."


Here's a use of 'modulo' by a mathematician working on the four-colour theorem quoted in Msrk Walters, "It Appears That Four Colors Suffice: A Historical Overview of the Four-Color Theorem (2004):

Shortly after testing the final configuration for reducibility, Appel celebrated the success by etching the statement ‘Modulo careful checking, it appears that four colors suffice’ onto the department’s blackboard.

This follows the general sense 'A mod B' being 'A seems generally true but for B' but B is not necessarily an exception, but something to bear in mind.


Like zero or orthogonal, the word modulo has a precise technical meaning in mathematics. This makes it useful in some limited non-mathematical situations. But most people just say it to sound smart, and miss the point.

Good usage

In mathematics, "A = B modulo C" means roughly that A and B are the same thing, except for differences of type C.

  • Butane and isobutane are the same, modulo their shape.
  • If you remember your dad's birthday but don't remember which year, then you know the day of his birth modulo one year.

Bad usage

Sometimes people use "modulo" as a synonym for "except for." Example: "All mammals, modulo the monotremes, give birth to live young." This example is arguably correct, but we have far better words for this (like except).

Sometimes people use the word modulo to mean "almost." This is almost as bad as saying "literally" when you mean "figuratively." The word "modulo" has nothing to do with closeness. In math, -1 and 999999999999 are equal modulo 1000000000000.

Your example

This proposal is the best so far, modulo the fact that parts of it need modification.

If the author knows what "modulo" really means, this should mean

This proposal would be the best so far if parts of it were modified.

The dictionary author could have found a better example.

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