The ion flow velocity entering the Debye sheath is constrained to satisfy the Bohm criterion: velocity must be equal to or greater than sonic. The ion flow velocity entering the magnetic pre-sheath (which arises upstream of the Debye sheath when a magnetic field, oblique to the solid surface is present) is constrained to satisfy the Chodura Criterion: velocity-parallel-to-B must be equal to or greater than sonic. The Bohm criterion, in its marginal form where v = cs, is associated with a "gentle entry" into the sheath with Δne = Δni, while v > cs corresponds to |Δne| > |Δni|. The magnetic pre-sheath, in contrast with Debye sheath, is a regin of quasi-neutrality and thus only Δne = Δni is permitted. This might appear to prohinbit supersonic flow parallel-to-B at the entrance to the magnetic pre-sheath, which the analysis of Chodura and Riemann has found to be permitted, in fact. This apparent contradiction is resolved by analysing the magnetic pre-sheath region to bring out more explicitly the role played by inertia in the E×B direction, thus confirming that such supersonic flow is permitted.