Diagonal neutral currents

General conditions are derived for neutral currents to be diagonal in all quark fields. The solution is a generalization of the Glashow-Iliopoulos-Maiani scheme to many quark fields with two distinct charges. It is shown that the diagonality and universality of the neutral operator is preserved by leading terms of the single-loop diagrams. In contrast to the basic assumption of this article, neutral-current charm-changing transitions lead to apparent $\left|\Delta S\right|=1\mathrm{}$ neutral currents, apparent lepton-number violation, and large $D^0-{\overline{D}}^0$ mixing. The article also describes a sum rule, which is independent of the number of quarks, and surveys the experimental status of charm-changing neutral currents.