In a contact analysis, you solve for the contact state and the contact forces. The most fundamental quantity in a contact analysis is the distance between the boundaries which may come into contact;,the gap. If the gap is positive then there is no interaction. The task of the contact algorithm is to ensure that the gap never becomes negative; that is to avoid overclosure. In order to accomplish that, contact forces must be introduced.

If friction is ignored, the state only consists of being in contact or not, and the force variable is the contact pressure in the normal direction. With friction included, there are three possible states for the relative tangential displacement when in contact: positive gap, sticking or sliding. The tangential force vector is added as force variable.