What exactly comprises a relative velocity in special relativity? I find it confusing when distant galaxies can be moving away from the earth at speeds approaching the speed of light and yet that separation speed contributes nothing to a relative velocity. The usual answer provided is that both the earth and the distant galaxies are motionless in their local space. Therefore their relative velocity is zero. However, this means that to correctly determine a relative velocity the differential motion of space between the two bodies also must be considered.