  At the tree level, the scattering processes involving open and closed strings
are described by a disk world-sheet with vertex operator insertions at the
boundary and in the bulk. Such amplitudes can be decomposed as certain linear
combinations of pure open string amplitudes. While previous relations have been
established on the double cover (complex sphere) in this letter we derive them
on the disk (upper complex half plane) allowing for different momenta of the
left- and right-movers of the closed string. Formally, the computation of disk
amplitudes involving both open and closed strings is reduced to considering the
monodromies on the underlying string world-sheet.
