A framed structure is perfect if it contains members equal to, where n = number of joints in a frame.
Perfect frame: If the number of members is just sufficient to keep the frame in equilibrium without a change in its shape under the action of external load. The perfect frame satisfies the following equation
m = 2n − 3
m = number of members in the frame
n = number of joints in the frame
In a Perfect frame, all of its support reactions and member forces can be calculated using only the equations of static equilibrium.