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It often happens that two or more similar values are distinguished with the ' symbol, e.g. a, a', a'' and similar. How is this pronounced?

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Note that through the joys of Unicode we now have an actual prime symbol so you can do a′ now instead of using an apostrophe for the job. There are even double prime ″ and triple prime symbols ‴. (OK there are still lots of times you might still have to use an apostrophe - the future ain't perfect yet.) –  hippietrail May 23 '11 at 1:57
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4 Answers

up vote 29 down vote accepted

A' would typically be called A prime; A'' would be called A double prime and so on.

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'Dash' is also reasonably common here in Cambridge, particularly when talking about differentials perhaps (f-dash). –  Nicholas Wilson May 23 '11 at 19:48
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As @waiwai933 said, the answer is A prime, A double prime, ans so on. As usual, Wikipedia has more details. In French (and probably other languages), double prime, triple prime, and so on are named seconde (2nd), tierce (an old word for 3rd). According to wikipedia, it was the same in English before the 1960s (but the relevant sentence is tagged with [citation needed]).

This progression is indeed the etymological origin of the symbol, which was initially a superscript Roman number.

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In my father's generation, this was "a dash" in the UK, but I think "a prime" has overtaken this since.

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I can't post comments yet, but isn't « A" » pronounced « A second » ?

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it's not a usage I'm familiar with, but see @Frédéric's answer above. All I can say for sure is that the "second" and "third/tierce" naming style wasn't something I heard as a mathematician in the late 70s or early 80s. –  user1579 May 23 '11 at 13:01
    
I skipped that one, sorry. Thanks for the heads up though ;) –  Isaac Clarke May 23 '11 at 14:03
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