Valve is an anatomical term applied to the shell of those molluscs that have a shell. Although in theory any mollusc shell can be termed a "valve", the word is now most commonly applied to members of two classes of molluscs: the Bivalvia (clams) and the Polyplacophora (chitons), in other words, to those molluscs whose shells are normally composed of more than one articulating part, each one of which is known as a valve (and also, in the case of chitons, as a "plate".)