As the nineteenth century wore on, other investigators, notably H. J. S. Smith and Hermann Minkowski, explored the application of Gauss's concept of the genus to higher-dimensional forms, and introduced some invariants for the genus from which in this century Hasse was able to obtain a complete and very simple classi¯cation of rational quadratic forms based on Hensel's notion of p-adic number", which has dominated the theory ever since.