  Relativistic charge distributions of targets with arbitrary average momentum
are introduced. They provide an interpolation between the usual Breit frame and
infinite-momentum frame distributions. We find that Breit frame distributions
can be interpreted from a phase-space perspective as internal charge
quasi-densities in the rest frame of a localized target, without any
relativistic correction. We show also that the apparent discrepancies between
Breit frame and infinite-momentum frame distributions simply result from
kinematical artifacts associated with spin.
