  The issue of the net charge at insulating oxide interfaces is shortly
reviewed with the ambition of dispelling myths of such charges being affected
by covalency and related charge density effects. For electrostatic analysis
purposes, the net charge at such interfaces is defined by the counting of
discrete electrons and core ion charges, and by the definition of the reference
polarisation of the separate, unperturbed bulk materials. The arguments are
illustrated for the case of a thin film of LaAlO$_3$ over SrTiO$_3$ in the
absence of free carriers, for which the net charge is exactly 0.5$e$ per
interface formula unit, if the polarisation response in both materials is
referred to zero bulk values. Further consequences of the argument are
extracted for structural and chemical alterations of such interfaces, in which
internal rearrangements are distinguished from extrinsic alterations (changes
of stoichiometry, redox processes), only the latter affecting the interfacial
net charge. The arguments are reviewed alongside the proposal of Stengel and
Vanderbilt [Phys. Rev. B {\bf 80}, 241103 (2009)] of using formal polarisation
values instead of net interfacial charges, based on the interface theorem of
Vanderbilt and King-Smith [Phys. Rev. B {\bf 48}, 4442 (1993)]. Implications
for non-centrosymmetric materials are discussed, as well as for interfaces for
which the charge mismatch is an integer number of polarisation quanta.
