In math mode, writing \prime gives too big a "prime" symbol, and it isn't a superscript, like it always is in textbooks. Why is this?

This isn't a question about how to get \prime to be a superscript, I know that. The questions is about the reasoning behind why it isn't like that by default.

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    Probably not what you're asking either, but ' produces a superscript “prime”. Maybe \prime is just the command that holds the symbol, and it's only intended to use for defining other commands (such as ')? – Juan A. Navarro Sep 1 '10 at 11:07

This is actually one of the exercises in The TeXBook :) (exercise 16.5, p. 130):

Why do you think TeX treats \prime as a large symbol that appears only in superscripts, instead of making it a smaller symbol that has already been shifted up into the superscript position?

And the answer is:

The second alternative doesn’t work properly when there’s a subscript at the same time as a prime. Furthermore, some mathematicians use \prime also in the subscript position; they write, for example, $F'(w,z)=\partial F(w,z)/\partial z$ and $F_\prime(w,z)=\partial F(w,z)/\partial w$.

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    subscripted primes!? I've never seen that. Still, I guess the problem with it not working well if there's a subscript is legitimate... – Seamus Sep 1 '10 at 12:45
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    Anyway, it seems the most natural choice, given that TeX has no "real" superscripts, just regular font (with proper optical size, if any) scaled down, so treating \prime the same way is natural in the sense that it fits with the rest of the system. – Khaled Hosny Sep 1 '10 at 13:06
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    I always thought that the \prime macro existed so that one can write expressions such as f^{\prime,n}. After reading your answer I realized that f'^n works fine. – Aditya Oct 1 '11 at 2:15

There are situations where you need the big \prime instead of small ' e.g. $J^{v\prime h}$ looks great, but $J^{v ' h}$ wont serve as good as the previous one.

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If the default is subscript, that would appear to make the regular form inaccessible, essentially removing functionality.

edit: this appears to mesh with a statement on wikipedia which suggests the prime symbol is big so that it can be optionally subscripted.

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