vtrticol or horirontd. Thus, the D4hplanor form has o four-fold principol oxis, i, (therefore DJ and a horizonhi plane of symmetry (therefore Dad to which the vertical planes of 3ymmetry k, k ' ; I, 1'; m, m', n, n' are subsidiary. (Vertical ploner indicated by their liner of intersection with the horizonto1 plane of the ring.) Did wn-planal has a two-fold principal axis, j, (therefore 4 1 and two diagonoi (vertical) ploner of symmetry (therefore Dpd or sometimes Val, o, 0'; p, p' thot bisect the angles between the two rub4diary two-bid axesq, q'; r, r'. The idter 3ymmetry elements also hold for the D2d planar form. Dihedral angle, 8 , is the angle between p l m e A, A', containing carbon ohms CL, C*, and Cd and plane ~.~' ,~~t~i~i~~ Cs,G and Cc Volume 40, Number 10, October 1963 / 505 Dm and Dl,,, in dynamic equilibrium.