  We experimentally characterize the positions of the diffraction maxima of a
phase grating on a screen, for laser light at oblique incidence (so-called
off-plane diffraction or conical diffraction). We discuss the general case of
off-plane diffraction geometries and derive basic equations for the positions
of the diffraction maxima, in particular for their angular dependence. In
contrast to previously reported work [Jetty et al., Am. J. Phys. 80, 972
(2012)], our reasoning is solely based on energy- and momentum conservation. We
find good agreement of our theoretical prediction with the experiment. A
detailed discussion of the diffraction maxima positions, the number of
diffraction orders, and the diffraction efficiencies is provided. We assess the
feasibility of an experimental test of the phenomenon for neutron matter waves.
