  It is exactly proved that the classical Rayleigh Theorem on inflectional
velocity instability is wrong which states that the necessary condition for
instability of inviscid flow is the existence of an inflection point on the
velocity profile. It is shown that the disturbance amplified in 2D inviscid
flows is necessarily 3D. After the break down of T-S wave in 2D parallel flows,
the disturbance becomes a type of spiral waves which proceed along the
streamwise direction. This is just the origin of formation of streamwise
vortices.
