it is common practice to design the linkage system so that the movement of all of the bodies are constrained to lie on parallel planes, to form what is known as a "planar linkage". it is also possible to construct the linkage system so that all of the bodies move on concentric spheres, forming a "spherical linkage". in both cases, the degrees of freedom of the link is now three rather than six, and the constraints imposed by joints are now "c" = 3 − "f".