An 'energy principle' derived previously by Tasso (1978) is applied to the stability of shaped large aspect ratio Tokamaks. Most of the previous results on MHD, resistive and universal-type instabilities can be recovered in a simple way that demonstrates the power of this formation. This allows 3-dimensional tearing modes for general cross-sections to be investigated. An exact proof of the dissipative universal instability is given at least within the assumption of adiabatic motion. The relation of this to current work on collisionless drift is discussed.