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Pressing a digit or other numeric key begins numeric entry using the minibuffer. The number is pushed on the stack when you press the <RET> or <SPC> keys. If you press any other non-numeric key, the number is pushed onto the stack and the appropriate operation is performed. If you press a numeric key which is not valid, the key is ignored.

There are three different concepts corresponding to the word “minus,”
typified by ‘`a-b`’ (subtraction), ‘`-x`’
(change-sign), and ‘`-5`’ (negative number). Calc uses three
different keys for these operations, respectively:
`-`, `n`, and `_` (the underscore). The `-` key subtracts
the two numbers on the top of the stack. The `n` key changes the sign
of the number on the top of the stack or the number currently being entered.
The `_` key begins entry of a negative number or changes the sign of
the number currently being entered. The following sequences all enter the
number *-5* onto the stack: `0 <RET> 5 -`, `5 n <RET>`,
`5 <RET> n`, `_ 5 <RET>`, `5 _ <RET>`.

Some other keys are active during numeric entry, such as `#` for
non-decimal numbers, `:` for fractions, and `@` for HMS forms.
These notations are described later in this manual with the corresponding
data types. See Data Types.

During numeric entry, the only editing key available is <DEL>.