A system comprising a rigid body, a foundation and 5 bars each having one end coupled to the rigid body via three dimensional pin and the other end coupled to the foundation via another three dimentional pin, has generally one degree of freedom allowing translation of the rigid body along a circular locus, provided that the lengths and directions of all the bars are the same as viewed in a projection on one plane. It has been proved that the number of the states in which the system has two degrees of freedom allowing rotation besides translation of the rigid body is from 0 to 4. It has also been proved that there is generally one straight line which intersects the 5 bars in one state of two degrees of freedom. A method to design a system which keeps having one degree of freedom is discovered.