(Oblique; Monosymmetric).
In this system two of the angles between the crystallographic axes are
right angles, but the third angle is oblique, and the axes are of
unequal lengths. The axis which is perpendicular to the other two is
taken as OY = b (fig. 62) and is called the ortho-axis or
ortho-diagonal. The choice of the other two axes is arbitrary; the
vertical axis (OZ = c) is usually taken parallel to the edges of a
prominently developed prismatic zone, and the clino-axis or
clino-diagonal (OX = a) parallel to the zone-axis of some other
prominent zone on the crystal. The acute angle between the axes OX and
OZ is usually denoted as [beta], and it is necessary to know its
magnitude, in addition to the axial ratios a : b : c, before the
crystal is completely determined. As in other systems, except the
cubic, these elements, a : b : c and [beta], are characteristic of the
substance. Thus for gypsum a : b : c = 0.6899 : 1 : 0.4124; [beta] =
80 deg. 42'; for orthoclase a : b : c = 0.6585 : 1 : 0.5554; [beta] =
63 deg. 57'; and for cane-sugar a : b : c = 1.2595 : 1 : 0.8782;
[beta] = 76 deg. 30'.