The lengths of the links comply with the conditions: B͞C=E͞D and C͞D=B͞E, i.e. figure BCDE is a parallelogram linkage. Rigid triangle HED is similar to triangle GHA. For any configuration of parallelogram linkage BCDE, triangle GHA retains constant angles at its vertices. When link 1 turns about fixed axis A, selected as the centre of similarity, and one of the points, G or H, 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, G or H, can be selected as the centre of similarity.
$847$LW,P$