The description of any waveguide includes its cut-off number, its propagation constant, one or more definitions of impedance, power flow and attenuation and a description of its field pattern. Since the rectangular wave guide embodies all the salient properties of the ridge waveguide it is apt to review some of its more important features before tackling the ridge structure. The chapter is restricted to a description of the dominant mode in the geometry but the existence of higher order modes is understood. The orthogonal property of any two modes of the waveguide is separately established.