When you need to force two or more degrees of freedom to take on the same (but unknown) value, you can couple the degrees of freedom together. A set of coupled degrees of freedom contains a prime degree of freedom, and one or more other degrees of freedom. Coupling will cause only the prime degree of freedom to be retained in your analysis' matrix equations, and will cause all the other degrees of freedom in a coupled set to be eliminated. The value calculated for the prime degree of freedom will then be assigned to all other degrees of freedom in a coupled set.

Typical applications for coupled degrees of freedom include: 1) maintaining symmetry on partial models, 2) forming pin, hinge, universal, and slider joints between two coincident nodes, and 3) forcing portions of your model to behave as rigid bodies. (See this chapter's discussion of constraint equations for more general rigid region capability.)