right-margin-width

right-margin-width is a variable defined in `C source code'.
Its value is

0

• Automatically becomes buffer-local when set.

Documentation:
Width in columns of right marginal area for display of a buffer.
A value of nil means no marginal area.

Setting this variable does not take effect until a new buffer is displayed
in a window. To make the change take effect, call `set-window-buffer'.