Citation: Wyckoff, R. W. G. (1931). page 274 from the second edition of Structure of Crystals by Wyckoff R W G. Published by The Chemical Catalog Company, INC, New York in 1931 http://jcrystal.com/steffenweber/gallery/StructureTypes/st4.html . The second edition of Structure of Crystals, , pp. 274-274.