A composite of two or more crystal individuals having a definite crystallographic relationship to each other. The orientation of one individual may be the mirror image of the other across a twin plane, or an orientation that can be derived by rotating the twin portions about a twin axis, or some other rational twin law. Twinned individuals in a twin crystal commonly show reentrant angles between crystal faces or on cleavage planes. A twin crystal may exhibit symmetry higher than that of its crystal individuals.