Figure 1: (a)—(c) show how the charges of the $\pi$ mesons from the $D^{*}$ and the $D$ decay can be used to tag the $D$ “flavor” ($D^0$ or ${\overline{D}}^0$) at production and decay. In a right-sign (RS) decay the charges match [see (a)], unlike in wrong-sign (WS) decays, for which there are two possible sources [see (b),(c)]. (d) displays the weak interaction process responsible for $D^0-{\overline{D}}^0$ oscillations, which involves all three generations of quarks.