The parameters are a : a : c, where a refers to the two equal
horizontal axes, and c to the vertical axis; c may be either shorter
(as in fig. 42) or longer (fig. 43) than a. The ratio a : c is spoken
of as the axial ratio of a crystal, and it is dependent on the angles
between the faces. In all crystals of the same substance this ratio is
constant, and is characteristic of the substance; for other substances
crystallizing in the tetragonal system it will be different. For
example, in cassiterite it is given as a : c = 1 : 0.67232 or simply
as c = 0.67232, a being unity; and in anatase as c = 1.7771.