GNU Emacs Manual: Transient Mark

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H.2 Transient Mark Mode

On a terminal that supports colors, Emacs can highlight the current region. But normally it does not. Why not?
Highlighting the region whenever it exists would not be desirable in Emacs, because once you have set a mark, there is always a region (in that buffer). And highlighting the region all the time would be a nuisance. So normally Emacs highlights the region only immediately after you have selected one with the mouse.
You can turn on region highlighting by enabling Transient Mark mode. This is a more rigid mode of operation in which the region "lasts" only temporarily, so you must set up a region for each command that uses one. In Transient Mark mode, most of the time there is no region; therefore, highlighting the region when it exists is useful and not annoying.
To enable Transient Mark mode, type M-x transient-mark-mode. This command toggles the mode, so you can repeat the command to turn off the mode.
Here are the details of Transient Mark mode:

To set the mark, type C-SPC (set-mark-command). This makes the mark active and thus begins highlighting of the region. As you move point, you will see the highlighted region grow and shrink.

The mouse commands for specifying the mark also make it active. So do keyboard commands whose purpose is to specify a region, including M-@, C-M-@, M-h, C-M-h, C-x C-p, and C-x h.

When the mark is active, you can execute commands that operate on the region, such as killing, indenting, or writing to a file.

Any change to the buffer, such as inserting or deleting a character, deactivates the mark. This means any subsequent command that operates on a region will get an error and refuse to operate. You can make the region active again by typing C-x C-x.

Commands like M-> and C-s, that "leave the mark behind" in addition to some other primary purpose, do not activate the new mark. You can activate the new region by executing C-x C-x (exchange-point-and-mark).

C-s when the mark is active does not alter the mark.

Quitting with C-g deactivates the mark.

Some commands operate on the region whenever it is active. For instance, C-x u in Transient Mark mode operates on the region when there is a region. Outside Transient Mark mode, you must type C-u C-x u if you want it to operate on the region. See section D.4 Undoing Changes. Other commands that act this way are identified in their own documentation.

The highlighting of the region uses the region face; you can customize the appearance of the highlighted region by changing this face. See section AD.2.2.3 Customizing Faces.
When multiple windows show the same buffer, they can have different regions, because they can have different values of point (though they all share one common mark position). Ordinarily, only the selected window highlights its region (see section O. Multiple Windows). However, if the variable highlight-nonselected-windows is non-nil, then each window highlights its own region (provided that Transient Mark mode is enabled and the mark in the window's buffer is active).
When Transient Mark mode is not enabled, every command that sets the mark also activates it, and nothing ever deactivates it.
If the variable mark-even-if-inactive is non-nil in Transient Mark mode, then commands can use the mark and the region even when it is inactive. Region highlighting appears and disappears just as it normally does in Transient Mark mode, but the mark doesn't really go away when the highlighting disappears.
Transient Mark mode is also sometimes known as "Zmacs mode" because the Zmacs editor on the MIT Lisp Machine handled the mark in a similar way.

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