Monoclinic system
The fourth system has all the axes unequal, the a axis and the b axis at right angles to each other, but the c axis is inclined to the a axis, meeting it at some other than a right angle. The monoclinic system is like the orthorhombic system except that it leans, or is askew, in one direction. The result is that the faces at the ends of the b axis are rhombohedral, while the others are rectangular. As in the foregoing system, the faces are in pairs at opposite ends of the axes; and as in the orthorhombic system, a face may occur on one edge and only have to be repeated on the edge diagonally opposite. The simplest form in this system will be made by combining the three pairs of faces at the opposite ends of the axes, which gives a prism, which is rectangular in cross section, but leans backward (or forward) if placed on end. As in all the systems, if a corner is cut, all must be cut; and if these corner faces are extended to meet each other, an octahedron results, in which, as in the prism, no two axes are equal. If this octahedron is properly orientated (i.e. with the a and b axes horizontal), it will lean forward or backward. Many minerals belong to this system; and, as in the orthorhombic system, it is not uncommon to have the vertical edges so beveled that two of the prism faces are obliterated, and the remaining two prism faces with the four new faces make a six-sided prism, which seems hexagonal. (See [plate 3], figure 3.) However, such a pseudo-hexagonal prism may be recognized by at least one pair of the faces having distinguishing marks (striæ, pits, or dullness), instead of all being just alike.