3.10.1 Highlighting the selection

Emacs has a concept of a mark and point that is similar to selections in other programs. But the mark in Emacs is used for more than just defining the selected region, it lives on while you continue to edit and move around the buffer so it can also be a kind of bookmark. The history of marks is saved so you can pop previous marks back to the top of the stack to go back to somewhere you were some time ago. Because of this dual purpose, the region between mark and point is not highlighted by default unless you select a region by clicking and dragging the mouse.

The minor mode transient-mark-mode changes the behavior of the mark in two ways. First, it distinguishes between an active mark that has just been defined or reactivated, and an inactive mark. When the mark is active, some commands that normally act on lines, words, buffers, etc., will instead act on the region. An inactive mark needs to be reactivated to operate on it, unless mark-even-if-inactive is set. Secondly, transient-mark-mode also highlights the region when it is active, providing the same visual clue that you get in other programs. This mode is turned on by default in latest versions of Emacs.

In addition to seeing the highlighting, new Emacs users often expect editing commands to replace the region when it is active. This behavior can be obtained with delete-selection-mode, but see the following question also.