# Book II

## Definition I.

A rectangle or a right angled parallelogram is ſaid to be contained by any two of its adjacent or conterminous ſides.

Thus: the right angled parallelogram is ſaid to be contained by the ſides and ; or it may be briefly deſignated by · .

If the adjacent ſides are equal; i.e. = , then · which is the expreſſion for the rectangle under and is a ſquare, and

is equal to
{
· or ^{2}
· or ^{2}

Q. E. D.