Euclid — "Rectilineal figures are those which are contained by straight lines..."

"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…"

"If a straight line be cut in extreme and mean ratio, the greater segment is also cut in extreme and mean ratio by the lesser segment."

"A semicircle is the figure contained by the diameter and the circumference cut off by it. And the center of the semicircle is the same as that of the circle."

"If equals be added to equals, the wholes are equal."

"Similar triangles are to one another in the duplicate ratio of their corresponding sides."