  We present a unified description of the low-energy limits of type II string
theories. This is achieved by a formulation that doubles the space-time
coordinates in order to realize the T-duality group O(10,10) geometrically. The
Ramond-Ramond fields are described by a spinor of O(10,10), which couples to
the gravitational fields via the Spin(10,10) representative of the so-called
generalized metric. This theory, which is supplemented by a T-duality covariant
self-duality constraint, unifies the type II theories in that each of them is
obtained for a particular subspace of the doubled space.
