  A proposed duality between type IIB superstring theory on R^9 X S^1 and a
conjectured 11D fundamental theory (``M theory'') on R^9 X T^2 is investigated.
Simple heuristic reasoning leads to a consistent picture relating the various
p-branes and their tensions in each theory. Identifying the M theory on R^{10}
X S^1 with type IIA superstring theory on R^{10}, in a similar fashion, leads
to various relations among the p-branes of the IIA theory.
