According to the principle of duality, a true boolean statement or expression can be converted to its dual form by replacing the “0”s in the statement or expression with “1”s, and vice versa and by replacing the “+”s in the statement or expression with “.”s, and vice versa.

The dual form of (X+Y)(X + Z).(Y + Z) is

(X.Y)+(X.Z)+(Y.Z)

In the above dual expression all the “+”s were replaced by “.”s, and vice versa.