A universal joint, sometimes called a Hooke or Cardan joint, is shown in the diagram below with the two bodies that it connects. A universal joint allows two relative rotations of the two connected frames, about orthogonal axes; this joint type prevents all of the other relative motions of the two frames. In the diagram below, the joint angles, α and β, represent the relative rotation of the end frame,, with respect to the start frame, . In this example, the first rotation, α, is about the axis, while the second rotation, β, is about the axis.
The universal joint can be considered as a composite joint comprised of two revolute joints, and an intermediate massless body can be considered the cross. The first rotation occurs about one of the axes of the cross and the second rotation occurs about the second axis.
