  In this paper, the stability of inviscid parallel flow between two parallel
walls is studied. Firstly, it is obtained that the profile of the base flow for
this classical problem is a uniform flow. Secondly, it is shown that the
solution of the disturbance equation is cr=U and ci=0, i.e., the propagation
speed of the disturbance equals the flow velocity and the disturbance in this
flow is neutral. Finally, it is suggested that the classical Rayleigh Theorem
on inflectional velocity instability is incorrect which states that the
necessary condition for instability of inviscid parallel flow is the existence
of an inflection point on the velocity profile.
