(Pyramidal; Quadratic; Dimetric).
In this system the three crystallographic axes are all at right
angles, but while two are equal in length and interchangeable the
third is of a different length. The unequal axis is spoken of as the
principal axis or morphological axis of the crystal, and it is always
placed in a vertical position; in five of the seven classes of this
system it coincides with the single tetrad axis of symmetry.
[Illustration: FIG. 42.]
[Illustration: FIG. 43.