Unix: what modifier key does “M-” refer to (e.g. M-C)

Question

I\'m trying to do a case-sensitive search in Nano. The help menu at the bottom of the screen says I can toggle case-sensitivity by typing M-C.

Answer 1

M refers to the meta key, which is not present on most keyboards. (Its use in software is for primarily historical reasons.) Usually, the meta key is emulated by another key on your keyboard. On Windows and Linux, it is usually the Alt key. On Mac OS X, that key (aka Option) already has other uses, and so Escape is used instead.