Research papers and textbooks addressing the problem of internal rotation in a molecule explain symmetry properties of the torsional potential by local geometrical symmetries of the molecule. It is shown here that symmetry properties of a torsional potential derive from permutation inversion symmetry and a peculiar nature of torsional dynamics but have no relation to actual geometrical symmetries. To confirm the validity of our symmetry analysis a minimum energy torsional potential curve has been determined ab initio for acetaldehyde, resulting in exact 2pi/3 periodicity that no previous ab initio calculations achieved.