"Further, of trilateral figures, a right-angled triangle is that which has a right angle, an obtuse-angled triangle that which has an obtuse angle, and an acute-angled triangle that which has its three…"

"That which is without parts has no magnitude."

"Trilateral figures are those contained by three straight lines, quadrilateral those contained by four, and multilateral those contained by more than four straight lines."

"A quantity is said to be a part of a quantity, the less of the greater, when it measures the greater."

"That all right angles are equal to one another."