[Brian_K_White]

It's immaterial if some key on some keyboard in some language happens to have a key that produces that glyph by composing with shift, or not.

If you asked how to type numbers, and I said "same way you type letters", that does not mean by typing "A" and wondering why it didn't produce "1".

The question was what is a compose key or what does compose key mean.

Shift is an example of a compose key that everyone is familar with, just no one calls it that. But everyone already understands what the shift key does. If you know what the shift key is and how to use it and what it does, then you know what a compose key is. It's that.

All the other possible compose keys just do exactly that same thing, just more of it for more and different glyphs and control codes.

[jgtrosh]

I understand you mean compose as a generic term for a symbol modifier? But for the compose key, the word composition is used as a synonym of combination, and that of a set of symbols; which is not the case of shifting each symbol into a corresponding set.

I agree that ctrl/alt/meta/shift are somewhat synonymous, and all those+compose are just instances of modifier keys.

But this combination meaning is specifically what makes the compose-key distinction useful and clear.

[Brian_K_White]

No not symbol modifier. Compose.

To get a particular glyph, or control code, you have to compose it out of multiple keypresses.

Some common examples happen to be modified versions of whatever glyph happens to be on a key, but that is just a subset, arranged that way for human sanity, humans would hate it if you had to press shift+g to get A, and really only in the latin languages whose writing even works that way. The glyphs could actually be anything, and outside of the latin languages they are. The poop emoji is not a modified form of any number or letter.

And it's not even just glyphs but any byte or byte sequence you want to produce. A single 0x01 byte is not a glyph, nor a modified form of any glyph, but you can still type it, and it's composed of the CTRL key and the A key.