The lengths of the links comply with the conditions: A͞B=D͞C, C͞B=A͞D, C͞B=G͞D and A͞B>B͞C. Figure A BCD is a crossed-crank linkage. Link 1, rotating about fixed axis A, is connected by a sliding pair to slider 5 and by turning pairs B to links 3 and 8. Link 4, rotating about fixed axis D, is connected by a sliding pair to slider 6 and by turning pair C to link 3. Sliders 5 and 6 are connected together by turning pair E. Link 9, turning about axis D, is connected by turning pairs G to link 8 and slider 2 which moves along axis Ee of link 7. Link 7 is connected by turning pair E to sliders 5 and 6. Centres A and D are placed at the foci of an ellipse. Then, when link 1 rotates, point E describes the ellipse, and straight line Ee envelops this ellipse.