Customs and institutions affect and are affected by economic relations and processes. The two-way interaction is particularly important in studying history where the scale of the temporal canvas ensures that very few variables can be treated as parametric. This paper assesses the methodology which attempts the task. In particular it examines the problem of endogenizing customs, evaluates claims for the optimality of institutions, and also comments on the interplay between structural and inertial forces. Recent work in the new institutional economics stresses structural forces, while traditional history emphasizes inertial forces, but on closer analysis these are shown to be complementary.