  We examine the general nature of nuclear odd-even mass differences by
employing neutron and proton mass relations that emphasize these effects. The
most recent mass tables are used. The possibility of a neutron excess
dependence of the staggering is examined in detail in separate regions defined
by the main nuclear shells, and a clear change in this dependency is found at
$Z=50$ for both neutrons and protons. A further separation into odd and even
neutron (proton) number produces very accurate local descriptions of the mass
differences for each type of nucleons. These odd-even effects are combined into
a global phenomenological expression, ready to use in a binding energy formula.
The results deviate from previous parametrizations, and in particular found to
be significantly superior to a recent two term, $A^{-1}$ dependence.
