The third system has all three axes unequal, but all three are still at right angles with each other. This is saying that the lines of force in the crystals are all at right angles to each other but of unequal value. The faces in this case are all in pairs. What happens at one end of an axis must happen at the opposite end, but does not need to happen at the ends of any of the other axes. We are dealing with pairs of faces (one at either end of an axis), and if three such pairs are combined in the simplest manner, the resulting figure will be a rectangular prism. If we cut the eight corners of this prism and enlarge the faces until they meet, the result is an octahedron, in which the distance from top to bottom, from side to side, or from front to back is not the same in any two cases. (See Plate 2.) In this system if a face is made by beveling one edge of the prism there must be a corresponding face on the edge diagonally opposite, but there does not have to be one on any of the other edges. However if a corner is cut, that face affects all the axes and so all the corners must be cut. A great many crystals occur in this system, and some of them which are prismatic in shape may give trouble, for it is not uncommon for the vertical edges of the prism to be so beveled, that two of the original prism faces are obliterated, and the two remaining faces added to the four new faces make a six-sided prism, which at first glance seems to belong to the hexagonal system. (See Plate 3, fig. 3.) Close examination however will show that, instead of all the prism faces being alike, as would be necessary for the hexagonal system, they are really in pairs, and one pair at least will be distinguished in some way, such as being striated, pitted, or duller. Monoclinic system