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11.7 Disabling Transient Mark Mode

The default behavior of the mark and region, in which setting the mark activates it and highlights the region, is called Transient Mark mode. This is a minor mode that is enabled by default. It can be toggled with M-x transient-mark-mode, or with the ‘Active Region Highlighting’ menu item in the ‘Options’ menu. Turning it off switches Emacs to an alternative mode of operation:

While Transient Mark mode is off, you can activate it temporarily using C-<SPC> C-<SPC> or C-u C-x C-x.

Set the mark at point (like plain C-<SPC>) and enable Transient Mark mode just once, until the mark is deactivated. (This is not really a separate command; you are using the C-<SPC> command twice.)
C-u C-x C-x
Exchange point and mark, activate the mark and enable Transient Mark mode temporarily, until the mark is next deactivated. (This is the C-x C-x command, exchange-point-and-mark, with a prefix argument.)

These commands set or activate the mark, and enable Transient Mark mode only until the mark is deactivated. One reason you may want to use them is that some commands operate on the entire buffer instead of the region when Transient Mark mode is off. Enabling Transient Mark mode momentarily gives you a way to use these commands on the region.

When you specify a region with the mouse (see Setting Mark), or with shift-selection (see Shift Selection), this likewise activates Transient Mark mode temporarily and highlights the region.