Proposition 16 . Proposition 17 . Proposition 18 . To describe a rectilinear figure similar and similarly situated to a given rectilinear figure on a given straight line. Proposition 19 .
Similar triangles are to one another in the duplicate ratio of the corresponding sides.
Proposition 20 . Similar polygons are divided into similar triangles, and into triangles equal in multitude and in the same ratio as the wholes, and the polygon has to the polygon a ratio duplicate of that which the corresponding side has to the corresponding side. Similar rectilinear figures are to one another in the duplicate ratio of the corresponding sides. Proposition 21 . Figures which are similar to the same rectilinear figure are also similar to one another.