Sign up ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

Sometimes I do read in a sentence:

some statement iff another statement

At first I thought it is a mispelling, but then I realized, that I do encounter this at many different posts. Does this mean equivalence?

share|cite|improve this question
Short answer: yes. –  Zhen Lin Jul 7 '11 at 12:10
Slightly longer answer: yes. Halmos claims to have invented it in his auto"math"ography I want to be a mathematician as well as the qed-box $\blacksquare$ (more a big black upright rectangle it was, but I don't know the TeX-code for that). –  t.b. Jul 7 '11 at 12:14
Relevant: you can write the if-and-only-if symbol $\iff$ in $\LaTeX$ with \iff. –  Samuel Jul 7 '11 at 15:07
@Samuel: Good to know, thanks! –  Aufwind Jul 7 '11 at 15:09
Just for interest $\phi$ 'just in case' $\theta$ is another way of saying $ \phi \iff \theta$ –  Paul Slevin May 28 '12 at 23:22

3 Answers 3

up vote 10 down vote accepted using google helps a lot sometimes.

share|cite|improve this answer
Generally you are right, but googling for iff alone is ambigious, see: [link][]. Ans since I am no native english speaker I didn't came up with if and only if. That's why I asked. And I am thankful for the short and the long answers. =) –  Aufwind Jul 7 '11 at 12:41
@ Aufwind: For me the fourth link is which directly leads you to the article i gave you :-) but no problem to ask. –  Listing Jul 7 '11 at 12:43
Damn, I see that right now, too. Fraunhofer and International Flowers and the list of all the other iffs must have distracted me too much... No offense. :-) –  Aufwind Jul 7 '11 at 12:47

As Zhen Lin already mentioned, "iff" is shorthand for "if and only if". Also in french literature one can find "ssi", which means "si et seulement si".

share|cite|improve this answer
There is also a video by Serre, where he says he is thankful to the germans, who do not use "dannn" instead of "dann und nur dann". –  Alexander Thumm Jul 7 '11 at 12:19
I think it's in his lecture on How to write mathematis badly, but I didn't have the patience to look for it in that video. –  t.b. Jul 7 '11 at 12:26
Thats exactly the one I meant. –  Alexander Thumm Jul 7 '11 at 12:31
J. H. Conway is always entertaining when he lectures. I recall that he got laughs when writing "unlesss" for "unless and only unless". –  GEdgar Jul 7 '11 at 14:48
ssi is also used in Spanish (si y sólo si). I seem to recall I've also seen people write sii instead. –  Andrés Caicedo Jul 7 '11 at 17:01

Using your example:

some statement iff another statement.

This is a short-hand way of combining the "if-then" form of a statement, and its converse or the vice versa statement when they are both true:

  • if another statement, then some statement;


some statement, if another statement (1)

  • if some statement, then another statement;


another statement, if some statement (2)

Combining (1) and (2) gives:

some statement iff another statement.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.