Comparison between Frames and Trusses:

Consider Figure, which is a truss. It consists of three members AB, BC and AC. It is supported by roller support at A and hinge support at C. The three members form a triangle. The load P acts at hinge B. The number of unknowns is six (forces in three members plus three reaction components). For three joints, three shall be six equations. This truss is geometrically stable and statically determinate. Figure shows a frame. It consists of two members AB and BC.

It is supported by the two hinge supports. The two hinge supports consist of four unknown reaction components. It seems as if the structure is indermate but it is not so. The free body diagrams of members AB and BC can be drawn as illustrated in Figure. Since members AB and BC are in equilibrium, there shall be three equations for AB and three equations of BC. In all there will be six equations. As shown in the Figure there are six unknowns only. Therefore, six equations can provide solution for six unknowns. Therefore, it is an example of find out structure. Figure also shows that members in frame need not make a triangle.