When a seismic reflector is a dipping plane and the subsurface can be approximated by a single bed, the velocity needed to stack CDP data is higher than the velocity with which the energy travels in the subsurface. The ratio of the stacking velocity to the energy-travel velocity increases from unity for strike lines to the secant of the dip angle for dip lines. Although the reflection points are not common, reflections stack even for steep dips. Stacking velocities for multiple reflections increase with the order of the multiple, but multiple reflections may also be stacked. To convert from reflection time to depth requires knowledge not given by the stacking velocity. In theory, a split-spread procedure can furnish the needed information.