9.3.1 Completion Example

A concrete example may help here. If you type M-x au <TAB>, the <TAB> looks for alternatives (in this case, command names) that start with `au'. There are several, including auto-fill-mode and auto-save-mode, but they all begin with auto-, so the `au' in the minibuffer completes to `auto-'.

If you type <TAB> again immediately, it cannot determine the next character; it could be any of `cfilrs'. So it does not add any characters; instead, <TAB> displays a list of all possible completions in another window.

Now type f <TAB>. This <TAB> sees `auto-f'. The only command name starting with that is auto-fill-mode, so completion fills in the rest of that. You have been able to enter `auto-fill-mode' by typing just au <TAB> f <TAB>.