DeadKeys

A dead key is a special kind of a modifier key on a typewriter or computer keyboard that is typically used to attach a specific diacritic to a base letter. The dead key does not generate a (complete) character by itself but modifies the character generated by the key struck immediately after.

For instance, when the United States-International (AltGr dead keys) layout is used, pressing !AltGr+' will do nothing. Pressing !AltGr+' followed by E would produce é.