23.3.5 Window Header Lines

Starting in Emacs 21, a window can have a header line at the top, just as it can have a mode line at the bottom. The header line feature works just like the mode line feature, except that it's controlled by different variables.

Variable: header-line-format

This variable, local in every buffer, specifies how to display the header line, for windows displaying the buffer. The format of the value is the same as for mode-line-format (see section 23.3.1 The Data Structure of the Mode Line).

Variable: default-header-line-format

This variable holds the default header-line-format for buffers that do not override it. This is the same as (default-value 'header-line-format).

It is normally nil, so that ordinary buffers have no header line.