Sometimes we use the word *or* (or the word *unless*) in such a way that knowing one option to be true immediately rules out the other (as in saying you will have either coffee or tea). The choices, then, are mutually exclusive, and we can symbolize this with the signal **M**. Often this is not necessary, as in the tautologous statement "*either you are studying or you are not*," which can be symbolized just as readily with the signal **O**.

Most systems of symbolic logic do not have special notation for mutual exclusion. Instead the concept has to be expressed by asserting the disjunction and denying the conjunction (as we would do in the string **PQOPQANA**).

The truth table for mutual exclusion is as follows:

**P Q || PQM
T T........F
T F........T
F T........T
F F........F**

One thing to note is that the string **PQM** is equivalent to the negation of **PQB**.

Return to variables and signals.

Go back to the starting page.