**(2)** The *square* of a relation is that relation which hold between two terms *x* and *z* when there is no intermediate term *y* such that the given relation holds between *x* and *y* and between *y* and *z*. Thus “paternal grandfather” is the square of “father”, “greater by 2” is the square of “greater by 1”, and so on.

Bertrand Russell, *Introduction to Mathematical Philosophy*, pp 32–3