  The doubly ionized $d^9$ copper ion provides, originally in La$_2$CuO$_4$ and
later in many more compounds, the platform for high temperature
superconductivity when it is forced toward higher levels of oxidation. The
nearest chemical equivalent is Ag$^{2+}$, which is almost entirely avoided in
nature. AgO is an illustrative example, being an unusual nonmagnetic insulating
compound with an open $4d$ shell on one site. This compound has been
interpreted in terms of one Ag$^{3+}$ ion at the fourfold site and one Ag$^{+}$
ion that is twofold coordinated. We analyze more aspects of this compound,
finding that indeed the Ag$^{3+}$ ion supports only four occupied $4d$-based
Wannier functions per spin, while Ag$^+$ supports five, yet their physical
charges are nearly equal. The oxygen $2p$ Wannier functions display two
distinct types of behavior, one type of which includes conspicuous Ag $4d$
tails. Calculation of the Born effective charge tensor shows that the mean
effective charges of the Ag ions differ by about a factor of two, roughly
consistent with the assigned formal charges. We analyze the $4d$ charge density
and discuss it in terms of recent insights into charge states of insulating
(and usually magnetic) transition metal oxides. What might be expected in
electron- and hole-doped AgO is discussed briefly.
