Geometry: Common Core (15th Edition)

Similar figures have corresponding angles that are congruent and corresponding sides that are proportional. All rectangles must have congruent angles because all angles of a rectangle are $90^{\circ}$. If the corresponding sides are shown to have the same scale factor, meaning the ratios of corresponding sides are all the same, then the two rectangles are similar. If the corresponding sides do not have the same scale factor, then the rectangles are not similar. Therefore, we can conclude that rectangles can sometimes be similar.