CAN any of your readers help me as to the original definition of the familiar term “pinacoid”? I suspect that it was introduced by C. F. Naumann about 1830; it was derived from πirαe;, a slab, and appears from the first to have included two parallel planes. Naumann, for instance (“Anfangsgriinde der Krystallographie,” 1841, p. 126), uses “basal pinacoid” for the pair of planes parallel to the two lateral crystallographic axes. But he restricts the use of pinacoid to the three possible pairs in a crystal that cut only one of the three axes, and (p. 19) defines a pinacoid as including “two parallel planes which are parallel either with the base or one of the other coordinate planes.”