I've just been asked by a singer I know if I can find out what concert key charts are, as he's been asked to provide them for some jazz musicians he might perform with. Unfortunately, I've never come across them, though I assume they're something like the Nashville number system which has often been used in recording studios for basic notation of songs.

If anybody could direct me to an explanatory website (though Google didn't give me much just now) or explain these terms, that would be appreciated. Thanks!