The fourbar mechanism is the quintessential example of what Reuleaux called a kinematic chain. In this planar mechanism, four links are constrained by four cylindrical joints. When one link is fixed or grounded, the motion of a second link determines the motion of the third and fourth links. Compare the motion of different inversions of this linkage in the movies C01, C01a and C01b, and notice how the motion has changed. This inversion is called "crankcrank". For mathematical description of the motion of these inversions see tutorial "Mathematics of four bar linkages".
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