(Rhombic; Prismatic; Trimetric).
In this system the three crystallographic axes are all at right
angles, but they are of different lengths and not interchangeable. The
parameters, or axial ratios, are a: b: c, these referring to the axes
OX, OY and OZ respectively. The choice of a vertical axis, OZ = c, is
arbitrary, and it is customary to place the longer of the two
horizontal axes from left to right (OY = b) and take it as unity: this
is called the "macro-axis" or "macro-diagonal" (from [Greek: makros],
long), whilst the shorter horizontal axis (OX = a) is called the
"brachy-axis" or "brachy-diagonal" (from [Greek: brachus], short). The
axial ratios are constant for crystals of any one substance and are
characteristic of it; for example, in barytes (BaSO4), a: b: c =
0.8152 : 1 : 1.3136; in anglesite (PbSO4), a: b: c = 0.7852: 1 :
1.2894; in cerussite (PbCO3), a : b : c = 0.6100 : 1 : 0.7230.