Shift key ignored in C-x Shift-o?

Question

I wanted to define a key binding C-x O (similar to C-x o but with a capital O). Before doing that, I tried it to see if it is already used. But it seems that Shift-o is just treated as o without holding the Shift. The echo line shows C-x o instead.

I have used key combinations of Shift with other modifiers, e.g. M-< goes to the beginning of buffer but M-, doesn't.

But when used alone, is Shift always ignored as in C-x S-o?

Also, this makes me wonder:

Is Shift key a modifier just as Control and Meta?; or is it not a modifier, but rather just used to change the character that follows, e.g. from , to < and o to O?

Also, I looked up the key C-x S-o with C-h k and got:

C-x o (translated from C-x O) runs the command other-window ...

It is bound to C-x o.

It did say, "translated from C-x O". Does this translation always happen? Would it be wrong if I go against it and define C-x S-o to do something else?

(This is with Emacs 28 in Ubuntu 22.04)

Answer 1

Is Shift key a modifier just as Control and Meta?; or is it not a modifier, but rather just used to change the character that follows, e.g. from , to < and o to O?

AFAIK it is both: pressing Shift and o will cause Emacs to see O with the Shift modifier bit set.

It did say, "translated from C-x O". Does this translation always happen?

This behavior is controlled by the variable translate-upper-case-bindings.

Would it be wrong if I go against it and define C-x S-o to do something else?

No, you wouldn’t be wrong, as the translation happens only if the lowercase binding exists but the uppercase one doesn’t. See C-h v translate-upper-case-bindings RET for more details.