Not all terminals have the complete set of modifiers. Terminals that have a Meta key allow you to type Meta characters by just holding that key down. To type Meta-a, hold down META and press a. On those terminals, the META key works like the SHIFT key. Such a key is not always labeled META, however, as this function is often a special option for a key with some other primary purpose.
If there is no META key, you can still type Meta characters using two-character sequences starting with ESC. To enter M-a, you could type ESC a. To enter C-M-a, you would type ESC C-a. ESC is allowed on terminals with Meta keys, too, in case you have formed a habit of using it.
If you are running under X and do not have a META key, it is possible to reconfigure some other key to be a META key. See Super and Hyper Keys.
Emacs believes the terminal has a META key if the variable
meta-flag is non-
nil. Normally this is set automatically
according to the termcap entry for your terminal type. However, sometimes
the termcap entry is wrong, and then it is useful to set this variable
yourself. See Variables, for how to do this.
Note: If you are running under the X window system, the setting of
meta-flag variable is irrelevant.