Compose key
The key that merges the next two presses into one character – Compose, apostrophe, e gives é – a DEC inheritance that X11 preserved as the keysym Multi_key.
DEC's LK201 keyboard for the VT220 in 1983 had a key labelled Compose Character: pressing it lit a lamp, and the next two keystrokes were combined according to a table inside the terminal. Sun put a Compose key with its own LED on its keyboards, and X11 adopted the idea as the keysym Multi_key. The sequences live in a file called Compose under /usr/share/X11/locale/, one per locale, and users can add their own in ~/.XCompose: <Multi_key> <minus> <greater> : "→".
Unlike dead keys, which only yield a handful of accents, Compose is a general character factory: Compose o c gives ©, Compose 1 2 gives ½, Compose s s gives ß, and anyone so inclined can define whole words. The interpretation happens in the client, originally in Xlib's input routine, and GTK and Qt carry their own implementations that read the same file. Few keyboards have the key today, so the XKB options compose:ralt and compose:menu borrow another one. Windows has no such feature without third-party tools like WinCompose, and macOS covers the same need with Option as a dead key. For those who once learned the sequences, it is one of the few things from 1980s terminals still missed on other platforms.