The rule of thumb to determine the handedness is: If an x- or s-face is present at the left side of an r-face, the quartz is called left-handed (or left quartz, for short). If an x- or s-face is present at the right side of an r-face, the quartz is called right-handed (or right quartz, for short). The figure shows the s-faces (tinted blue) and x-faces (tinted orange) on both left- and right-handed quartzes and their relative position to the r-, z-, and m-faces. Left and right quartz are mirror images of each other, but lack mirror symmetry themselves. Double-terminated crystals with all s- and x-faces on them like those on the idealized renderings are extremely rare. Most crystals don't show s- and x-faces, and those that do often have only one or two of them, and either x- or s-faces, but rarely both of them.