Dead keys

Dead keys come from mechanical typewriters: a dead key prints an accent, but does not advance the carriage like normal keys do; that's why it's called "dead". Therefore the next character is printed in the same position, resulting in a letter with an accent above it.

On those mechanical typewriters the vertical position of the dead key accents is fixed obviously, on the correct height for lowercase letters but not high enough for uppercase letters. The typewriter won't stop you (and can't stop you) from using a dead key in combination with uppercase letters, but the result will not be satisfactory.

Computers are smarter than mechanical typewriters and can produce the correct glyph, with the correct vertical position of the accent to match the letter it's combined with. On computers the user experience is different though: when you press a dead key, nothing seems to happen. No output appears. Only on the next key press is output generated. What that output looks like depends on the combination. For example, if I press ´ followed by a, I get á. But if I press ´ followed by space I simply get ´, and combined with z it gives me ´z (because z with an acute accent doesn't exist, I guess).

Belgian azerty has a number of dedicated keys for the most common letters with accents, so you don't need to use dead keys for those (though you could, if you wanted to): é è à ù (ù doesn't seem all that common to me though; I would think ê is more common). All other accented characters require the use of one of the dead keys (^ ¨ ´ ` ~). ^ is a special case in that it appears twice on the keyboard: once as a dead key to produce e.g. ê , and once as a normal key to produce ^ (which I can also produce by first pressing the dead key ^ followed by space, just as I can with all the other dead keys). I don't know why ^ is special enough to get two appearances.

Side note: "Dedicated" is not entirely the correct term here: all those keys produce other letters when combined with Shift, and often also with AltGr. For example, é is on the same key as 2 (Shift) and @ (AltGr) (yes, azerty keyboards require Shift to type digits, which is why people using azerty have a somewhat higher tendency to use the numerical keypad for numerical entry).

Second side note: The term "dead key" is not exactly correct either, since being dead or not is not a feature of the key itself anymore like it was on mechanical typewriters. For example the key with ^ produces a very normal non-dead [ when used with AltGr. There are no fully dead keys anymore (on Belgian azerty, at least).

(That's probably more than you wanted to know about dead keys and accents on azerty keyboards)