AltGr and other layered layouts put a third symbol on the same physical key, switched by a modifier

What it is

Layouts in Europe and elsewhere add AltGr (often right Alt) besides left and right Alt. Held, it makes the same physical key emit a third character set: currency, bracket variants, €, or @ on some maps. Shift is level two (capitals and shifted symbols); AltGr is level three; AltGr+Shift is sometimes level four. A level is a table lookup while the modifier is held, not a two-stroke dead-key sequence. Caps often print the third level in a corner in small type; the user must know which level is active.

Why it happens

The driver indexes layout levels 0/1/2/3 with the scan code plus the current modifier bitmap. On many systems AltGr equals Ctrl+Alt, so application shortcuts on Ctrl+Alt+letter collide with third-level characters. Laptops often merge AltGr with right Alt to save a key; a US user who plugs in a European board suddenly gains a layer of unknown characters. Unlike a dead key, a level does not change whether this stroke emits immediately, only which character comes out. Unlike ordinary Shift, level three is infrequent and printed smaller, so it is easier to forget.

Studying it

Have local and visiting users type currency signs and programming brackets on an AltGr layout, and record whether they use level three, an insert panel, or copy-paste. Audit whether app shortcuts occupy Ctrl+Alt+letter. Legend readability can take viewing distance and lighting as factors. Do not evaluate European third levels only with US-board participants; that treats “never seen AltGr” as a layout defect.

Where it stops holding

US ANSI layouts usually have no meaningful third level; AltGr, if present, is rarely taught. Soft keyboards replace levels with paged symbol panels. Games that bind right Alt as its own action kill the third level. Touch typists who ignore legends carry the third level as pure memory load. While an IME is composing, AltGr may be swallowed, making level three unstable in a Chinese state.

Applying it

• If a product needs €, @, or square brackets, test that they are typeable on European layouts, and avoid making Ctrl+Alt plus the same letter a non-substitutable shortcut.
• On-screen keyboard maps should draw corner characters and mark the AltGr-only level, not only the primary character.
• Remote desktop and VMs must pass AltGr through rather than treating it as ordinary Alt.
• Verify on the target regional layout, with no character panel, that every special mark in the product docs is typeable; list those that need AltGr, then check that application shortcuts have not stolen those chords.

Related

• Same group: C6.21.1 A dead key does not emit a character immediately; it combines with the next key to form a diacritic · C6.21.2 Without a cue for the pending dead-key state, people think the key is dead · C6.21.4 Layered layouts raise per-key information density and the chance of hitting the wrong layer
• Adjacent: C6.04 Modifier keys · C6.18 Regional keyboard layout differences
• Search: AltGr · keyboard level · third-level symbols