A variable‐metric (VM) crystal is one in which the spacing between the crystalline planes changes with position in the crystal. This variation can be either parallel to the crystalline planes or perpendicular to the crystalline planes of interest and can be produced by either introducing a thermal gradient in the crystal or by growing a crystal made of two or more elements and changing the relative percentages of the two elements as the crystal is grown. A series of experiments were performed in the laboratory to demonstrate the principle of the variable‐metric crystal and its potential use in synchrotron beam lines. One of the most useful applications of the VM crystal is to increase the number of photons per unit bandwidth in a diffracted beam without losing any of the overall intensity. In a normal synchrotron beam line that uses a two‐crystal monochromator, the bandwidth of the diffracted photon beam is determined by the vertical opening angle of the beam which is typically 0.10–0.30 mrad or 20–60 arcsec. When the VM crystal approach is applied, the bandwidth of the beam can be made as narrow as the rocking curve of the diffracting crystal, which is typically 0.005–0.050 mrad or 1–10 arcsec. Thus a very large increase of photons per unit bandwidth (or per unit energy) can be achieved through the use of VM crystals. When the VM principle is used with bent crystals, new kinds of x‐ray optical elements can be generated that can focus and defocus x‐ray beams much like simple lenses where the focal length of the lens can be changed to match its application. Thus both large magnifications and large demagnifications can be achieved as well as parallel beams with narrow bandwidths.