49.3.7 Modifier Keys

The default key bindings in Emacs are set up so that modified alphabetical characters are case-insensitive. In other words, C-A does the same thing as C-a, and M-A does the same thing as M-a. This concerns only alphabetical characters, and does not apply to shifted versions of other keys; for instance, C-@ is not the same as C-2.

A Control-modified alphabetical character is generally considered case-insensitive: Emacs always treats C-A as C-a, C-B as C-b, and so forth. The reason for this is historical: In non-graphical environments there is no distinction between those keystrokes. However, you can bind shifted Control alphabetical keystrokes in GUI frames:

(global-set-key (kbd "C-S-n") #'previous-line)

For all other modifiers, you can make the modified alphabetical characters case-sensitive (even on non-graphical frames) when you customize Emacs. For instance, you could make M-a and M-A run different commands.

Although only the Control and Meta modifier keys are commonly used, Emacs supports three other modifier keys. These are called Super, Hyper, and Alt. Few terminals provide ways to use these modifiers; the key labeled Alt on most keyboards usually issues the Meta modifier, not Alt. The standard key bindings in Emacs do not include any characters with the Super and Hyper modifiers, and only a small number of standard key bindings use Alt. However, you can customize Emacs to assign meanings to key bindings that use these modifiers. The modifier bits are labeled as ‘s-’, ‘H-’ and ‘A-’ respectively.

Even if your keyboard lacks these additional modifier keys, you can enter it using C-x @: C-x @ h adds the Hyper flag to the next character, C-x @ s adds the Super flag, and C-x @ a adds the Alt flag. For instance, C-x @ h C-a is a way to enter Hyper-Control-a. (Unfortunately, there is no way to add two modifiers by using C-x @ twice for the same character, because the first one goes to work on the C-x.)