Polarization measurements are deemed complete when measurement at an additional polarization yields no new information. This condition is satisfied, in completely polarized scatter, by measurements of the four elements of the scattering matrix; this can be accomplished by transmitting a particular polarization and its orthogonal. The corresponding measurements for partially polarized radiation are the elements of the coherency matrix. This matrix does not yield diagonal product terms, nor are these terms redundant. They are readily obtained, however, by the transmission of a third polarization. Examples relating circular and linear polarizations are given.