There are also two sets of three C2 axes in the plane of the snowflake, one set
through opposite points and one through the cut-in regions between the points. One of
each of these axes is shown in Figure 4-2. In molecules with more than one rotation axis,
the C, axis having the largest value of n is the highest order rotation axis or principal
axis. The highest order rotation axis for a snowflake is the C6 axis. (In assigning Cartesian
coordinates, the highest order C, axis is usually chosen as the z axis.) When necessary,
the C2 axes perpendicular to the principal axis are designated with primes; a single
prime (C2') indicates that the axis passes through several atoms of the molecule, whereas
a double prime (C2") indicates that it passes between the outer atoms.