Compile emacs mode extensions

Occasionally I want to compile a new mode for emacs but I can never remember the command so here it is:

M-x byte-compile-file [RET]
This is: press the “Esc” key (top left on my keyboard), then the letter x, then type “byte-compile-file” and press the return key.

or to compile all .el files in a directory:

[ESC] x byte-recompile-directory [RET]