This paper contains sample calculations and a description of the plastic design of a square knee for a single-span rigid frame. Welds are designed using the concepts of plastic design as well as those of elastic design in order that the parallelism can be seen. The methods of design presented here can be used as a guide in designing comparable connections for rectangular portal frames. A theoretical analysis of a straight knee with diagonal stiffeners is also presented and leads to expressions for the reinforcement required within the knee to prevent undue deformation. A further analysis is made of the rotation and deflection of the connection.