The hexad axis is uniterminal and all the forms are open forms.
This is the hexad axis or hexagonal axis, for which the angle of rotation is 60, or one-sixth of 360.
Perpendicular to the hexad axis are six dyad axes of two kinds and each perpendicular to a vertical plane of symmetry.
In crystals of this division of the hexagonal system the principal axis is a hexad axis of symmetry.
Here there is a plane of symmetry perpendicular to the hexad axis; there is also a centre of symmetry.
No other element is here associated with the hexad axis, which is uniterminal.
Six planes of symmetry of two kinds intersect in the hexad axis.
Six dyad axes of two kinds are perpendicular to the hexad axis.
There can be only one hexad axis of symmetry in any crystal (see figs. 77-80).