A method is presented for giving a precise meaning to the concept of a "normal mode" in dimethylacetylene for the limiting case of almost free internal rotation. The normal modes so defined are shown to transform according to irreducible representations of the molecular symmetry group discussed in a previous paper. In addition, the effect on the energy levels of Coriolis interaction between the vibrational motion and both the overall rotation and the torsion is discussed for doubly degenerate and quadruply degenerate vibrational states. Selection rules for transitions between the various Coriolis components are presented, which are analogous to the "(+l), (−l) selection rules" for "ordinary" symmetric-top molecules. The effect of a small barrier to internal rotation is discussed briefly.