The meeting point of lines that connect corresponding points from homothetic figures. In the above figure, O is the homothetic center of the homothetic figures A B C D E and A' B' C' D' E'. For figures which are similar but do not have parallel sides, a similitude center exists. Given two nonconcentric circles, draw radii parallel and in the same direction. Then the line joining the extremities of the radii passes through a fixed point on the line of centers which divides that line externally in the ratio of radii. This point is called the external homothetic center, or external center of similitude (Johnson 1929, pp. 19-20 and 41).

Apollonius' problem | complement | homothecy | homothetic | perspective | similitude center | similitude ratio