  We show how the duality invariant approach to M-theory formulated by Berman
and Perry relates to the double field theory proposed by Hull and Zwiebach. In
doing so we provide suggestions as to how Ramond fields can be incorporated
into the double field theory. We find that the standard dimensional reduction
procedure has a duality invariant (doubled) analogue in which the gauge fields
of the doubled Kaluza-Klein ansatz encode the Ramond potentials. We identify
the internal gauge index of these gauge fields with a spinorial index of
O(d,d).
