Emacs is complicated enough that it needs more commands than Exercise 9.17 allows. To allow for more commands, Emacs has been extended, as follows. Meta+X and Control+X—as in eXtended—are command...

Emacs is complicated enough that it needs more commands than Exercise 9.17 allows. To allow for more commands, Emacs has been extended, as follows. Meta+X and Control+X—as in eXtended—are command prefixes, so that neither Meta+X nor Control+X is a valid command, but, for example, “Control+X Control+U” is (and it’s different from Control+U). A valid command can be formed by Control+X or Meta+X followed by any letter or any command character (including Control+X or Meta+X). All other command characters from Exercise 9.17 are still valid.

How many command characters are there now?

Exercise 9.17

How many command characters are there in Emacs? Emacs is complicated enough that it needs more commands than Exercise 9.17 allows. To allow for more commands, Emacs has been extended, as follows. Meta+X and Control+X—as in eXtended—are command prefixes, so that neither Meta+X nor Control+X is a valid command, but, for example, “Control+X Control+U” is (and it’s different from Control+U). A valid command can be formed by Control+X or Meta+X followed by any letter or any command character (including Control+X or Meta+X). All other command characters from Exercise 9.17 are still valid.