Crystallographic axes: the crystallographic axes are rects selected in a way to form between each other identical angles to those formed by the three edges of the crystal. In certain cases it is appropriate to choose a four axes system. It is agreed to name ax X the ax facing the observer; ax Y the ax crossing left to right the crystal in parallel to the observer; ax Z the ax bottom to top. When axes are 4, the fourth ax is indicated by W and is placed upright while the remaining three are perpendicular or differently inclined with respect to the ax W. The crystallographic axes, while intersecting, are divided in a positive and in a negative part.
