  We consider supergravity in twelve dimension, whose dimensional reduction
yields eleven-dimensional, IIA, and IIB supergravities. This also provides the
effective field theory of F-theory. We must take one direction as a compact
circle, so that the Poincar\'e symmetry and the zero-mode field contents are
identical to those of eleven-dimensional supergravity. We also have a tower of
massive Kaluza-Klein states to be viewed as the wrapping modes of M2-branes.
The twelfth dimension decompactifies only if other two directions are
compactified on a torus, restoring different ten dimensional Poincar\'e
symmetry of IIB supergravity, whose missing graviton is provided by components
of the rank three tensor field. This condition prevents us from violating the
condition on the maximal number of real supercharges, which should be
thirty-two. The self-duality condition of the IIB four-form fields is
understood from twelve-dimensional Hodge duality. In this framework T-duality
is re-interpreted as taking different compactification routes.
