decrease-right-margin is an interactive compiled Lisp function in `indent.el'.

It is bound to <C-down-mouse-2> <in> <decrease-right-margin>, <menu-bar> <edit> <props> <in> <decrease-right-margin>.

(decrease-right-margin FROM TO INC)

Make the right margin of the region smaller.
With no prefix argument, decrease the right margin by `standard-indent'.
A prefix arg (optional third arg INC noninteractively) specifies the amount
of width to remove, in characters. A negative argument increases
the right margin width.
If `auto-fill-mode' is active, re-fills region to fit in new margin.