The lengths of the links comply with the conditions: B͞C=A͞D and A͞B=D͞C, i.e. figure ABCD is a parallelogram linkage. Rigid triangle ECB is similar to triangle EFA. For any configuration of parallelogram linkage ABCD, triangle EFA retains constant angles at the vertices. When link 1 turns about fixed axis A, selected as the centre of similarity, and one of the points, E or F, is traced along any arbitrary path, the other point describes a similar path turned through a constant angle. The mechanism has reversibility since any point, A, E or F, can be selected as the centre of similarity.
$836$LW,P$