If you check the key bindings of a shifted key, and the shifted key is not bounded to anything, Emacs will silently translate it to the unshifted key – if and only if the unshifted key is undefined.

On my system, both ‘f5’ and ‘S-f5’ are undefined. (Note that ‘S-f5’ is shift f5, while ‘s-f5’ is super f5!) Let’s check:

Now let us define f5:

 (global-set-key (kbd "<f5>") 'ignore)

Check again:

In this situation it is easy to be confused. But don’t worry, ‘S-f5’ is not “unrecognized by Emacs”. This silent translation is a feature! :)

 (global-set-key (kbd "S-<f5>") 'ding)

Check again:

Mystery solved!

Whoops, there is another such page, apparently: TheMysteriousCaseOfShiftedFunctionKeys. Sorry for the duplication. -- AlexSchroeder.

Here’s how to bind control-shift-v; first, the naive attempt:

    (global-set-key (kbd "C-V") 'somefunction)

This will bind both uppercase and lowercase C-v. Here’s the shifted version:

    (global-set-key (kbd "C-S-V") 'otherfunction)

In X, some keys get special treatment. The TAB key, for example, will suddenly report as S-iso-lefttab when shifted:

    (global-set-key (kbd "C-S-<iso-lefttab>") 'samefunction)