Key sequences longer than one key (and some single-key sequences) are written inside quotes or on lines by themselves, like this:
M-x frobnicate-while-foo <RET>
Any real spaces in such a key sequence should be ignored; only <SPC> really means press the space key.
The ASCII code sent by C-x (except for C-?) is the value that would be sent by pressing just x minus 96 (or 64 for upper-case X) and will be from 0 to 31. On Unix and GNU/Linux terminals, the ASCII code sent by M-x is the sum of 128 and the ASCII code that would be sent by pressing just x. Essentially, <Control> turns off bits 5 and 6 and <Meta> turns on bit 71.
C-? (aka <DEL>) is ASCII code 127. It is a misnomer to call C-? a “control” key, since 127 has both bits 5 and 6 turned ON. Also, on very few keyboards does C-? generate ASCII code 127.
 DOS and Windows terminals don't set bit 7 when the <Meta> key is pressed.