11 The Mark and the Region
Many Emacs commands operate on an arbitrary contiguous part of the current buffer. To specify the text for such a command to operate on, you set the mark at one end of it, and move point to the other end. The text between point and the mark is called the region. The region always extends between point and the mark, no matter which one comes earlier in the text; each time you move point, the region changes.
Setting the mark at a position in the text also activates it.
When the mark is active, we say also that the region is active; Emacs
indicates its extent by highlighting the text within it, using the
region face (see Customizing Faces).
This is one of the few faces that has the :extend t attribute
by default, which implies that the same face is used to highlight the
text and space between end of line and the window border. To
highlight only the text you could set this attribute to nil.
After certain non-motion commands, including any command that
changes the text in the buffer, Emacs automatically deactivates
the mark; this turns off the highlighting. You can also explicitly
deactivate the mark at any time, by typing C-g
(see Quitting and Aborting).
The above default behavior is known as Transient Mark mode. Disabling Transient Mark mode switches Emacs to an alternative behavior, in which the region is usually not highlighted. See Disabling Transient Mark Mode.
Setting the mark in one buffer has no effect on the marks in other
buffers. When you return to a buffer with an active mark, the mark is
at the same place as before. When multiple windows show the same
buffer, they can have different values of point, and thus different
regions, but they all share one common mark position. See Multiple Windows.
Ordinarily, only the selected window highlights its region; however,
if the variable highlight-nonselected-windows is
non- nil, each window highlights its own region.
There is another kind of region: the rectangular region. See Rectangles.