  The pressure dependence of the order parameter in superfluid $^3$He is
amazingly simple. In the Ginzburg-Landau regime, i.e. close to $T_c$, the
square of the order parameter can be accurately measured by its proportionality
to NMR frequency shifts and is strictly linear in pressure. This behavior is
replicated for superfluid $^3$He imbibed in isotropic and anisotropic silica
aerogels. The proportionality factor is constrained by the symmetry of the
superfluid state and is an important signature of the corresponding superfluid
phase. For the purpose of identifying various new superfluid states in the
$p$-wave manifold, the order parameter amplitude of $^3$He-A is a useful
reference, and this simple pressure dependence greatly facilitates
identification.
