Figure 2
A structure on 4 circles with distance constraints in the plane can be modeled as a bar-and-joint structure with universal joints placed at the center of each circle and fixed-length bars between them. (a) Flexible framework with 4 bars and 1 degree of freedom can move to the dotted structure. Collisions are allowed; the circles for 2 and 4 overlap. (b) A 5th bar stabilizes the framework, making it rigid (with 0 degrees of freedom).