text: Internal coordinates are of the following types, illustrated with reference to the planar molecule ethylene, 
  * Stretching: a change in the length of a bond, such as C–H or C–C
  * Bending: a change in the angle between two bonds, such as the HCH angle in a methylene group
  * Rocking: a change in angle between a group of atoms, such as a methylene group and the rest of the molecule.
  * Wagging: a change in angle between the plane of a group of atoms, such as a methylene group and a plane through the rest of the molecule,
  * Twisting: a change in the angle between the planes of two groups of atoms, such as a change in the angle between the two methylene groups.
  * Out-of-plane: a change in the angle between any one of the C–H bonds and the plane defined by the remaining atoms of the ethylene molecule. Another example is in BF when the  boron atom moves in and out of the plane of the three fluorine atoms.
  In a rocking, wagging or twisting coordinate the bond lengths within the groups involved do not change. The angles do. Rocking is distinguished from wagging by the fact that the atoms in the group stay in the same plane.
  In ethylene there are 12 internal coordinates: 4 C–H stretching, 1 C–C stretching, 2 H–C–H bending, 2 CH rocking,  2 CH wagging, 1 twisting. Note that the H–C–C angles cannot be used as internal coordinates as well as the H-C-H angle because the angles at each carbon atom cannot all increase at the same time.
  Note that these coordinates do not correspond to normal modes (see #Normal coordinates). In other words, they do not correspond to particular frequencies or vibrational transitions.