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. Triclinic system The fifth or triclinic system has all the axes unequal, and no two of them intersect at right angles. As in the two preceding systems the faces occur in pairs at the opposite ends of the axes. This is the most difficult system in which to orientate a crystal, but fortunately only a few crystals occur in this system, such as the feldspars. Hexagonal system Lastly there is a group of crystals which have four axes, one vertical, and three in the horizontal plane which intersect each other at angles of 60°, all these three being equal to each other, but different from the vertical axis. The simplest form in this system is the six-sided prism. If one corner of this prism is cut all must be, and if these corner faces are extended to meet each other, a double-six-sided pyramid results. In this system if one of the vertical edges of the prism is beveled, all must be, but the horizontal edges need not be; or the horizontal edges may be beveled and the vertical ones not. The ends as they are related to the c axis may be developed independently of the prism, and so the prism may be simply truncated by a flat end, or have pyramids on either end. Hemihedral forms