Usage
A dead key is different from a typical modifier key (such as AltGr or ⌥ Option) in that, rather than being pressed and held while another key is struck, the dead key is pressed and released before striking the key to be modified. In some computer systems, there is no indication to the user that a dead key has been struck, so the key appears dead, but in some text-entry systems the diacritic is displayed along with an indication that the system is waiting for another keystroke to complete the typing sequence.
On a typewriter, the character modifier functionality is accomplished mechanically by striking the diacritical mark without advancing the carriage (in modern terms, that diacritical mark keys are non-spacing). Thus, the following letter will strike the same spot on the paper. By construction, this has no restrictions on a typewriter, so one could place an acute accent (´) on a q, for example.
Computers do not, however, work this way. On a computer, the dead key temporarily changes the mapping of the keyboard for the next keystroke, so it activates a special keyboard mode rather than generates a modifier character. Instead of the normal letter, a precomposed variant of it with the appropriate diacritic is generated. Each combination of a diacritic and a base letter must be specified in the character set and supported by the font in use. As there is no precomposed character to combine the acute accent with the letter q, striking ´ and then q is likely to result in ´q, with the accent and letter as separate characters, or in some systems the invalid typing sequence may be discarded silently. (Using the combining characters available in the Unicode character set, it may be possible to generate a combination that more or less looks like a q with an acute accent – q́ – but this is a technique quite distinct from the dead key functionality.)
Read more about this topic: Dead Key
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