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1.4 The Prefix Argument

The prefix argument is an integer argument which may be supplied to a command. It is known as the prefix argument because it is specified by invoking some prefix argument setting command immediately before the command to be given the argument. The following statements about the interpretation of the prefix argument are true:

• When it is meaningful, most commands interpret the prefix argument as a repeat count, causing the same effect as invoking the command that many times.
• When it is meaningful, most commands that use the prefix argument interpret a negative prefix argument as meaning the same thing as a positive argument, but the action is done in the opposite direction.
• Most commands treat the absence of a prefix argument as meaning the same thing as a prefix argument of one.
• Many commands ignore the prefix argument entirely.
• Some commands do none of the above.

The following commands are used to set the prefix argument:

Command: Argument Digit (bound to all control or meta digits) ¶

Typing a number using this command sets the prefix argument to that number, for example, typing M-1 M-2 sets the prefix argument to twelve.

Command: Negative Argument (bound to M–) ¶

This command negates the prefix argument, or if there is none, sets it to negative one. For example, typing M– M-7 sets the prefix argument to negative seven.

Command: Universal Argument (bound to C-u) ¶

Hemlock Variable: Universal Argument Default (initial value 4) ¶

This command sets the prefix argument or multiplies it by four. If digits are typed immediately afterward, they are echoed in the echo area, and the prefix argument is set to the specified number. If no digits are typed then the prefix argument is multiplied by four. C-u - 7 sets the prefix argument to negative seven. C-u C-u sets the prefix argument to sixteen. M-4 M-2 C-u sets the prefix argument to one hundred and sixty-eight. C-u M-0 sets the prefix argument to forty.

Universal Argument Default determines the default value and multiplier for the Universal Argument command.