But Emacs also has commands that take two or more events to invoke. If a sequence of events is enough to invoke a command, it is a complete key . If it isn't long enough to be complete, we call it a prefix key .
Every key sequence is either a complete key or a prefix key.
Most single characters constitute complete keys in the standard Emacs command bindings. A few of them are prefix keys. A prefix key combines with the following input event to make a longer key sequence, which may itself be complete or a prefix. There's no limit to the length of a key sequence, but in practice people rarely use sequences longer than four events. By contrast, you can't add more events onto a complete key.