Keyboard: What's the Difference Between Alt Graph Key, Compose Key, Dead Key?

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sun keyboard ret
The ◆ Meta, ⎄ Compose, AltGr keys, on Sun Microsystem's Type 6 Keyboard

What's the difference between {AltGr key, ⎄ Compose key, dead key}?

they are all used to enter special characters. The difference is how you press them.

Alt Graph Key

The Alt Graph key, usually labeled AltGr. This key is a modifier key. It is for inserting special characters. Press 【AltGr+‹key›】 to insert a special char.

For example, when using US international layout, press:

This key is on some European keyboards. It's also on Sun Microsystem's Type 6 Keyboard.

Which character is inserted with what key is dependent on the keyboard layout. 〔➤ International Keyboard Layouts

On Windows, the right Alt becomes AltGr if you set your keyboard layout to one of European layouts.

On the Mac, the ⌥ Opt key serve the same purpose as AltGr. 〔➤ Mac OS X Keyboard Viewer and Unicode

mac unicode char
〔➤ Mac OS X Keyboard Viewer and Unicode

⎄ Compose Key

The ⎄ Compose key is also for inserting special characters. But it is used in a different way than AltGr.

The ⎄ Compose is used as a starting key of a key sequence. Press ⎄ Compose first (and release it), then type other keys, to insert a special char. For example

The ⎄ Compose is on Sun Microsystem's Type 6 Keyboard and usually on unix “workstation” keyboards of 1980s.

Dead Key

Microsoft 4000 keyboard French dead key
Microsoft Natural Ergonomic Keyboard 4000, French version. The ^ and ¨ (to the right of P) are dead keys. (Full keyboard 1280×749) (photo by Alexander Sidorov.)
France kbd
French layout. The red ^ is a dead key. File:KB France.svg

Dead key is a special key similar to ⎄ Compose key. It is used for inputting chars with diacritic marks, such as:

You press the dead key first, then type a letter.

Many European language layout have both AltGr key and dead keys. See: International Keyboard Layouts.

On the Mac, 【⌥ Opt+e】 is a software dead key. It adds the acute mark to any letter typed after.

〔➤ Accent Marks: Trema, Umlaut, Macron, Circumflex, and All That

Emacs has over 1k keybinding, including a full set of keys for AltGr, ⎄ Compose, and dead keys. 〔➤ A Curious Look at GNU Emacs's 1000+ Default Keybinding

Which is Better Design? Alt Graph, Compose, or Dead Key?

AltGr and ⎄ Compose serves the same purpose. The only difference is that one is a modifier key, you need to hold it down. Compose is a better design, because:

Dead key has a special place. That is, if there are a few special characters that are used very frequently, having a dead key for them is more efficient than using ⎄ Compose. For example, suppose in your language the characters é and ú and í occur frequently. If you add dedicated keys for them, that would be expensive, because keys space is limited. Using ⎄ Compose, you might press 【⎄ Compose e '】. Using Dead key, you just press 【dead ' e】. Of course, you will need a extra physical key on your keyboard for each possible accent mark. But, if there's just a few specific symbol that you need to input frequently, it's good to have one dedicated dead key for it.

In general, keyboard should just have a ⎄ Compose key.

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