23.15.1 Interaction with normal keymaps

The end of a key sequence is detected when that key sequence either is bound to a command, or when Emacs determines that no additional event can lead to a sequence that is bound to a command.

This means that, while input-decode-map and key-translation-map apply regardless of whether the original key sequence would have a binding, the presence of such a binding can still prevent translation from taking place. For example, let us return to our VT100 example above and add a binding for C-c ESC to the global map; now when the user hits C-c PF1 Emacs will fail to decode C-c ESC O P into C-c PF1 because it will stop reading keys right after C-c ESC, leaving O P for later. This is in case the user really hit C-c ESC, in which case Emacs should not sit there waiting for the next key to decide whether the user really pressed ESC or PF1.

For that reason, it is better to avoid binding commands to key sequences where the end of the key sequence is a prefix of a key translation. The main such problematic suffixes/prefixes are ESC, M-O (which is really ESC O) and M-[ (which is really ESC [).