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I'm not a native English speaker. A quick Google search revealed the symbol's name is apostrophe, just like in French. When used in a mathematical setting, I usually call it prime, so for instance $f'$ I'll read $f$ prime. That's the way you call it in French, but I just realized I didn't actually know whether that was the way you call it.

Thank you!

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I am guessing it is region dependent. I am more familiar with "dash" when talking about derivative. So $f'(x)$ is f-dash x. The pronunciation could also vary with context, I suppose: when it means something other than derivative etc... – Aryabhata Jun 24 '11 at 6:03
This is part of a larger issue: Some symbols are pronounced using their name, and some are pronounced, at least in certain circumstances, using a special name. Besides the appostrophe being pronounced "prime" (when used to indicate a derivative), there are also the cases of the ampersand (&) being read "and" and the asterisk (*) being read "star". – Mike Jones Jun 25 '11 at 22:09
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Strictly speaking, the prime (′) is a different character from the apostrophe ('). – Rahul Narain Sep 2 '11 at 9:04
I would also read it as f prime. – user12205 Sep 2 '11 at 11:44
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@Arjang: Not at all! If I have $f$, I can write $f(x)$, $f(y)$, $f(5)$, $f(a+b)$, and so on. Why should just $x$ go in the denominator as though it were a free variable? To be precise, $f$ is a function mapping reals to reals and has nothing to do with some $x$, unless you write $\frac{df(x)}{dx}$ as you properly should. But the prime is better, because it doesn't require you to introduce a name for the argument of $f$. Then I can write $f'(5)$ for the derivative of $f$ evaluated at $5$, instead of $\left.\frac{df(x)}{dx}\right|_{x=5}$ (yuck)! – Rahul Narain Sep 2 '11 at 12:02
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2 Answers

up vote 18 down vote accepted

It varies with the region.

"Prime" is how you pronounce it in American English. Here is a nice reference for American English pronounciations of math symbols; $f'$ is on the top of the second page.

"Dash" is how you pronounce it in British English: Here (search for dash).

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And apparently the British English one is different :-) uefap.com/speaking/symbols/symbols.htm (and concurs with what I have been using all my life!) – Aryabhata Jun 24 '11 at 6:07
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In Israel we use "tag" (read like "tug"). – Yuval Filmus Jun 24 '11 at 6:08
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It may have been common to pronounce $f'$ as 'f dash' in British English in 1981, but for as long as I've been learning mathematics (including eight years in a British university) I've always called it 'f prime'. – Chris Taylor Jun 24 '11 at 8:08
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@Chris: I agree. "Dash" is better reserved for the en dash and em dash---those useful punctuators that you can't find on your keyboard. – John Bentin Jun 24 '11 at 8:24
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At school (in England) we were taught 'f dash', 'f double dash', and so on, but at soon as I got to University (also in England) it became 'f prime'. I haven't heard anyone say 'f dash' in a long time. – Alex Jun 24 '11 at 12:14
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up vote -1 down vote

$f'\rightarrow$ f prime

$f^{\prime\prime}\rightarrow$ f bis

$f^{\prime\prime\prime}\rightarrow$ f tris

$f^{\prime\ \backprime\prime}\rightarrow$ f tetrakis

However most people do not know these words and just say double prime, triple prime etc.

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Are these real names? I've done a quick google search but couldn't find any reference to any of these. – Olivier Bégassat Jan 5 at 15:16
I don't like this; prime is derived from Latin, whereas the others are from Greek. – user50229 Jan 5 at 15:17
@-1 Why?, do you think I made this up? – UnkleRhaukus Jan 28 at 14:13

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