Figure 19 (a) Example of crystal twinning, in which a twin plane separates two regions of a single crystal that are mirror images. The two regions are referred to as twin domains. (b) An example of simple twinning. (c) An example of multiple twinning. (d) Two crystals of orthoclase feldspar showing simple twinning. The left-hand one is 5 cm long. (e) Grain boundaries are different from twin boundaries because there is no orientation relationship between crystals on either side of the grain boundary. There are two distinct grains, with the same, or a different, mineral composition.