Last updated at 10:01 pm UTC on 31 March 2017
Middle button (usually) on a three button mouse: Activate a menu to invoke an action concerting the contents of a window; e.g. carrying out an editing operation on text withing the window.
If you have less than three buttons on your mouse, see FAQ: Mouse Buttons for more info on mappings.
Andrew P. Black on mouse button colors and the command key