Crystal Systems
With all the possible forms that crystals can and do take, there are six systems of arrangement. First there is the case where ranks, files, and vertical rows are all equal, and now to be scientific, instead of talking about ranks, files, etc., we use the term axes to express these ideas; the files or arrangements from front to back, being called the a axis, the ranks, or side to side arrangement the b axis, and the vertical arrangement the c axis. (See [Plate 1].) These axes are imaginary lines, but they represent real forces.