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. Proposition 22 . Proposition 23 .
Equiangular parallelograms have to one another the ratio compounded of the ratios of their sides.
Proposition 24 . In any parallelogram the parallelograms about the diameter are similar both to the whole and to one another. Proposition 25 . To construct a figure similar to one given rectilinear figure and equal to another. Proposition 26 .