Emulate3Buttons
The setting that gave a two-button mouse a pretend middle button, triggered by pressing both at once.
X inherited three mouse buttons from the Xerox workstations and used all of them: the middle one pastes the current selection, and many window managers hung menus off it. PC mice long had only two. So you put Option "Emulate3Buttons" in the pointer section of XF86Config, and pressing left and right within a short window — Emulate3Timeout, 50 milliseconds by default — counted as a middle click.
It worked, but every ordinary click paid a small delay while the server waited to see whether the other button was coming too. Clickable scroll wheels made the option rare, yet libinput keeps the same idea under the name middle button emulation, mostly for touchpads.