The lengths of the links comply with the conditions: D͞B=F͞C and B͞C=D͞F, i.e. figure DBCF is a parallelogram linkage. Point F lies on the straight line through points A and G of links 1 and 3. For any configuration of parallelogram linkage DBCF, points A, F and G lie on a single straight line. When link 1 turns about fixed axis A, selected as the centre of similarity, and one of the points, F or G, is traced along any arbitrary path, the other point describes a similar path. The mechanism has reversibility since any point, A, F or G, can be selected as the centre of similarity.
$841$LW,P$