9.4.2 Mode Line Formatting

Mode line formatting variables (e.g., gnus-summary-mode-line-format) follow the same rules as other, buffer line oriented formatting variables (see Formatting Basics) with the following two differences:

1. There must be no newline (‘\n’) at the end.
2. The special ‘%%b’ spec can be used to display the buffer name. Well, it’s no spec at all, really—‘%%’ is just a way to quote ‘%’ to allow it to pass through the formatting machinery unmangled, so that Emacs receives ‘%b’, which is something the Emacs mode line display interprets to mean “show the buffer name”. For a full list of mode line specs Emacs understands, see the documentation of the mode-line-format variable.