When generating your model, you typically define the relationships among different degrees of freedom by using elements to connect the nodes. However, you sometimes need to be able to model distinctive features (rigid regions, pinned structural joints, sliding symmetry boundaries, periodic conditions, and other special internodal connections) which cannot be adequately described with elements. You can establish such special associations among nodal degrees of freedom by using coupling and constraint equations (CEs). Using these techniques enables you to link degrees of freedom in ways that elements cannot.