The authors define an ab initio electronic structure model that uses partial spin restriction. It is an intermediate case between the so-called spin-restricted and spin-unrestricted formulations, which are popular in electronic structure methodology. Partial spin restriction arises naturally when the wave function is represented as an antisymmetrized product of two-electron functions, as it is done in generalized valence bond and antisymmetrized product of strongly orthogonal geminal theories. The authors show that the new model is size consistent, and it improves the description of transition metal compounds.