A truss is a structure that consists of triangular units composed of linear rigid members whose ends are connected at joints. All the joints in a truss are revolutes: the external forces and their reactions act only on the joints and create tensile or compressive forces in the members.
The Cremona diagram is a graphical method used in the static analysis of trusses to determine the forces on each of the members. Basically, this is an assembly of all the polygons representing the equilibrium of forces acting on each joint. This method was created by the Italian mathematician Luigi Cremona.
This Demonstration shows the Cremona diagram of a simple truss with seven members connected by five joints. Change the external load with the slider or move the two upper joints with the red locators. The free-body diagram of the truss and the Cremona diagram of the member forces adapt interactively. Each triangle represents the equilibrium at a joint. The circled number of the joint is in the center of the corresponding triangle.