In order to illustrate these points, let us consider an idealized ethanal molecule which is depicted in Figure and whose Z-matrix is written in Table . We choose two different orders of references for the H 4 atom attached to the carbonyl group. In the first case the position of the H 4 atom is defined by the angle (H 4 , C 2 , C 1 ), while in the second it is defined by the angle (H 4 , C 2 , O 3 ). If the angle (O 3 , C 2 , C 1 ) is modified it then leads to different structural changes of the molecule which is illustrated in Figure . These two different structural changes are not equivalent under translation/rotation and are depicted with different colors in Figure .