Ah initio calculations are reported which partition the energetics of internal rotation for ground state acetaldehyde according to potential type (repulsive and attractive), symmetry (sigma and pi), region (atomic basin), and nuclear virial (rotational path). We conclude that the origin of the barrier is complex, involving interplay of kinetic and potential energies of pi and sigma electrons. The formation of the barrier can be divided into three conceptual steps. (1) Rigid rotation occurs where the skeleton is frozen at the eclipsed equilibrium geometry during methyl rotation. This first step frequently leads to reasonable barrier heights, but it engenders increased kinetic energy of the pi electrons. (2) To relieve the repulsive pi-nuclear virial, the molecule relaxes by C-C bond lengthening, This lengthening causes both sigma and pi core energies to increase. But the greater sensitivity of sigma electrons to C-C bond lengthening overwhlems the pi contribution. (3) Other skeletal and methyl flexings, necessary to achieve the fully relaxed barrier top, correct the energetics governing (1) + (2). The outcome of all three steps is that changes in sigma orbitals through an increase in sigma nuclear-electron attraction energy contribute to the fully relaxed barrier. The pi-fragment model for internal rotation energetics considers just the first step, which neglects skeletal flexing.