6.3.1 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 are all the same as far as auto, so the ‘au’ in the minibuffer changes to ‘auto’.

If you type TAB again immediately, there are multiple possibilities for the very next character—it could be any of ‘c-’—so no more characters are added; instead, TAB displays a list of all possible completions in another window.

If you go on to type -f TAB, this TAB sees ‘auto-f’. The only command name starting this way is auto-fill-mode, so completion fills in the rest of that. You now have ‘auto-fill-mode’ in the minibuffer after typing just au TAB f TAB. Note that TAB has this effect because in the minibuffer it is bound to the command minibuffer-complete when completion is available.