  We consider the boundary states which describe D-branes in a constant B-field
background. We show that the two-form field Phi, which interpolates commutative
and noncommutative descriptions of D-branes, can be interpreted as the
invariant field strength in the T-dual picture. We also show that the extended
algebra parametrized by theta and Phi naturally appears as the commutation
relations of the original and the T-dual coordinates.
