Parallel to the base of the crystal (001).
Parallel to the faces of a cube (e.g., Halite).
Parallel to the faces of a dodecahedron.
Parallel to the faces of an octahedron (e.g., Fluorite).
Parallel to the faces of a prism.
Parallel to the faces of a rhombohedron.
The geometric orientation of planes along which a crystal tends to break.