What's the difference between {AltGr key, ⎄ Compose key, dead 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〕
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 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〕
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.