For the purpose of calculation of diffraction from building edges, we define a "Wedge" as having two faces, the 0-face and the ''n''-face. The wedge angle is a = (2-''n'')p, where the parameter ''n'' is required for the calculation of diffraction coefficients. All the diffracted rays lie on a cone with its vertex at the diffraction point and a wedge angle equal to the angle of incidence in the opposite direction. A diffracted ray is assumed to originate from a virtual image source point. Three triplets of unit vectors are defined as follows:
* [[File:frml19_tn.png]] <math>\mathbf{(\hat{u}_0, \hat{u}_l, \hat{t})}</math> representing the unit vector normal to the edge and lying in the plane of the 0-face, the unit vector normal to the 0-face, and the unit vector along the edge, respectively.* [[File:frml17_tn.png]] <math>\mathbf{(\hat{u}_f, \hat{u}_b, \hat{t})}</math> representing the incident forward polarization vector, incident backward polarization vector and incident propagation vector, respectively.* [[File:frml18_tn.png]] <math>\mathbf{(\hat{u}_f', \hat{u}_b', \hat{t}')}</math> representing the diffracted forward polarization vector, diffracted backward polarization vector and diffracted propagation vector, respectively.
The three triplets constitute three orthonormal basis systems. The propagation vector '''k'''' of the diffracted ray has to be constructed based on the diffraction cone as follows:
:<math> \mathbf{\hat{k}'} = \cos\phi_w \mathbf{\hat{u}_0} + \sin\phi_w \mathbf{\hat{u}_l} +
\mathbf{(\hat{k} \cdot \hat{t}) \hat{t}}, \quad 0 \le \phi_w \le \alpha</math>
[[File:frml8.png]]