  The generalized metric is a T-duality covariant symmetric matrix constructed
from the metric and two-form gauge field and arises in generalized geometry. We
view it here as a metric on the doubled spacetime and use it to give a simple
formulation with manifest T-duality of the double field theory that describes
the massless sector of closed strings. The gauge transformations are written in
terms of a generalized Lie derivative whose commutator algebra is defined by a
double field theory extension of the Courant bracket.
