A dead key does not emit a character immediately; it combines with the next key to form a diacritic

What it is

A dead key is a prefix in the layout: pressing it inserts no visible character, then waits for the next stroke and composes an accented letter, for example ´ then e yields é. It is not a modifier—it need not be held—and not an ordinary character key: a lone press usually leaves nothing (or inserts the accent only on a second dead-key press). Spanish, French, and Nordic layouts cover a full orthography with dead keys on a finite key count.

Why it happens

After dead-key down, the IME or system layout enters a compose state and looks up the next stroke in an “accent × base letter” table. A hit emits a precomposed character or a combining sequence; a miss emits the accent then the letter, or swallows the stroke, depending on the platform. A dead key is thus a two-keystroke mini-IME, with state hung on the next key rather than on whether a modifier is held. Unlike an AltGr layer, which must be held to change the current key's third character, a dead key is two strokes in sequence. Unlike CJK composition, the candidate space is tiny and almost deterministic; there is no paging.

Studying it

Give people unfamiliar with the layout a dead-key task list (accented e, nasal a, diaeresis u) and record whether they treat the dead key as an ordinary mark and mash it, and whether a failed compose inserts a bare accent. Independent variables include whether the cap marks the dead key and the fallback on compose failure. Dependent measures are one-shot compose success and extra characters. Mixing dead-key tasks with “hold Shift for capitals” in one test confuses prefix state with a quasi-mode.

Where it stops holding

US English layouts barely use dead keys; first contact with an international layout is interpreted through a press-equals-character model. Soft keyboards often put diacritics on a long-press popup and skip dead-key timing. Linux Compose is a longer prefix chain with similar rules but unbounded length. Apps that intercept keys themselves can break dead-key state so composition works only in system text fields. Screen readers that announce an empty dead-key stroke as “no input” make the key sound broken.

Applying it

• For multilingual desktop input, honor system dead keys; do not insert them as ordinary characters in a key listener.
• Custom input widgets should use the system text-input protocol so composition finishes at the framework before the app sees it.
• Print dead keys with a different mark from ordinary characters on caps and on-screen hints so they are not read as quotes or as the accent itself.
• Verify by typing every accented letter the product must support on the target layout, then an illegal follower (dead key then a digit), and confirm the failure fallback does not scramble the caret or submit a form.

Related

• Same group: C6.21.2 Without a cue for the pending dead-key state, people think the key is dead · C6.21.3 AltGr and other layered layouts put a third symbol on the same physical key, switched by a modifier · C6.21.4 Layered layouts raise per-key information density and the chance of hitting the wrong layer
• Adjacent: C6.04 Modifier keys · C6.25 IME state and switching
• Search: dead key · diacritic · compose key