Although a planar pair might theoretically be included, this would impose no constraint and thus be equivalent to an opening in the kinematic chain.
尽管理论上还可以包括平面，但平面却不能加以约束，因此只相当于开式运动链。
2
With the linear graph and adjacent matrix method of graph theory, the possible forms of this mechanism are discussed by analyzing that of kinematic chain with 4~8 links connected by planar low pair.