The principal goal of this contract USAF Contract F33615-70-C-1820 is to develop a semi-automated system for computing the radar cross section RCS of aerospace vehicles over the frequency range of 500-20,000 MHZ. Such a system requires the use of efficient techniques for calculating the high-frequency scattering from bodies with edges such as wings and ducts. In calculating the scattering from three-dimensional bodies with edges, it is frequently meaningful and useful to consider the scattering associated with an incremental length of the edge and to describe this scattering in terms of an Incremental Length Edge Diffraction Coefficient ILEDC. In this report the theory of the ILEDC is developed, taking into account the actual distribution of surface current near the edge. The theory is illustrated by applying it to the problem of scattering from a perfectly conducting polygonal plate. The Incremental Length Diffraction Coefficient ILDC, which is the generalization of the ILEDC for linear scattering features other than edges, is also treated. It is shown that two- dimensional diffraction coefficients can be considered as special cases of ILDCs.