Out of curiosity, is there a verb, which need not be popular, such that it says "suffice" and "require" simultaneously? I am after a verb that is equivalent to "if and only if".

The major context that originated this problem is a mathematics one. For example, a minimal workable one, the equivalence "The equation x+3=5 holds if and only if x = 2" can be said in another way as "for the equation x + 3 = 5 to hold requires that x = 2 and, for the equation x+3=5 to hold, it suffices that x = 2" (as one can easily check).

The most preferred verb of such a kind is a one that can be used to directly replace "if and only if" everywhere. I mean, if V is such a verb, then "x+3=5 if and only if x = 2" should be equivalent to "x+3=5 V x=2".

  • There might be - give us more context, a use-case. Jul 10, 2018 at 16:51
  • @Tetsujin. Good point. Added.
    – Yes
    Jul 10, 2018 at 16:54
  • First of all, I believe that only if is sufficient; although if and only if is a common expression, it's actually redundant and can be simplified. Assuming you agree, why would you need a single-word verb rather than only if? In other words, what's wrong with using only if that this verb would better address? Jul 10, 2018 at 16:59
  • 2
    ahh... maths is different. "If & only if" can be abbreviated to 'IFF' - see en.wikipedia.org/wiki/If_and_only_if Jul 10, 2018 at 17:03
  • 3
    The existence of the (relatively) common legal expression necessary and sufficient suggests that there is no single word for this, otherwise the law would use that instead.
    – Andrew
    Jul 10, 2018 at 17:06

3 Answers 3


Is there a verb meaning “suffice” and “require” simultaneously?
If V is such a verb, then "x+3=5 if and only if x = 2" should be equivalent to "x+3=5 V x=2".

The answer to this question is No. There is no such verb that means "is true only when".

  • Proof of your assertion?
    – Yes
    Jul 10, 2018 at 17:15
  • I just now combed through every verb in the English language and none of them meant "is true only when". The basis for your belief that such a verb might exist?
    – TimR
    Jul 10, 2018 at 17:17
  • I am neutral to my proposition "is there such a verb?", in fact. I just want to mention that it is also possible the "no" comes from the limitedness of knowledge at a certain time point.
    – Yes
    Jul 10, 2018 at 17:19
  • 3
    From the origin of the English language (as recorded) up until this very moment, there has never been such a verb. I cannot speak for the future.
    – TimR
    Jul 10, 2018 at 17:21

Such a verb inherently doesn't exist that would work in the context you are presenting. Grammatically, the mathematical expressions you are attempting to link are clauses, specifically independent clauses of the form "[subject] [verb]s [object]". Purdue's guidelines on clauses indicate that independent clauses can only be linked by either a conjunction or a special marking word. Using a verb to connect the independent clauses "x+3=5" and "x=2" would not be grammatical. The correct way to link those would be a conjunction. "If and only if" works as a conjunction here, but if you want one that's less formal other conjunctions that are close include "only when", "exclusively when", and "whenever".

If you want a word that works in a different context, a different example might be useful.


I'm going to quote Wikipedia, even though it's not something I would normally do:

In logic and related fields such as mathematics and philosophy, if and only if (shortened iff) is a biconditional logical connective between statements.

In that it is biconditional (a statement of material equivalence), the connective can be likened to the standard material conditional ("only if", equal to "if ... then") combined with its reverse ("if"); hence the name. The result is that the truth of either one of the connected statements requires the truth of the other (i.e. either both statements are true, or both are false). It is controversial whether the connective thus defined is properly rendered by the English "if and only if", with its pre-existing meaning. There is nothing to stop one from stipulating that we may read this connective as "only if and if", although this may lead to confusion.

In writing, phrases commonly used, with debatable propriety, as alternatives to P "if and only if" Q include Q is necessary and sufficient for P, P is equivalent (or materially equivalent) to Q (compare material implication), P precisely if Q, P precisely (or exactly) when Q, P exactly in case Q, and P just in case Q. Some authors regard "iff" as unsuitable in formal writing; others use it freely. . . .

The corresponding logical symbols are "↔", "⇔", and "≡", and sometimes "iff". These are usually treated as equivalent. However, some texts of mathematical logic (particularly those on first-order logic, rather than propositional logic) make a distinction between these, in which the first, ↔, is used as a symbol in logic formulas, while ⇔ is used in reasoning about those logic formulas (e.g., in metalogic). In Łukasiewicz's notation, it is the prefix symbol 'E'.

But after all of that, I agree with the other answer that, with respect to normal English, the simple answer is "No. There is no single verb."

  • We are fully aware of the common shorthands of math notation routines as a math apprentice. This does not stop anyone to inquire more possibilities.
    – Yes
    Jul 10, 2018 at 17:27

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