  We give a concrete characterization of the rational conjugacy classes of
maximal tori in groups of type $D_n$, with specific emphasis on the case of
number fields and p-adic fields. This includes the forms associated to
quadratic spaces, all of their inner and outer forms as well as the Spin
groups, their simply connected covers. In particular, in this work we handle
all (simply connected) outer forms of $D_4$.
