An Incompletely specified function is a Boolean function that only define output values for a subset of its inputs - i.e. a Boolean function whos output is a don't care for at least one of its input combinations. Incompletely specified functions often make no guarantees as to the unspecified output whatsoever.

Overview

A	B	C
0	0	0	1
0	0	1	X
0	1	0	X
0	1	1	1
1	0	0	1
1	0	1	0
1	1	0	0
1	1	1	X

In many situations when working with combinational circuits, some combinations of inputs should not occur under normal working conditions. For circuits with such combinations, those combinations can be treated as either 0 or 1 depending on whichever yields a more simplified Boolean expression.

Boolean functions with don't care output terms are represented with a . For example consider the following function.

Which can be represented with the following truth table and K-map.

Note that depending on the final design of the circuit, the output for for example may be either 0 or 1.