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.　in　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.