Twinning

n.
  1. The assemblage of two or more crystals, or parts of crystals, in reversed position with reference to each other in accordance with some definite law; also, rarely, in artificial twinning (accomplished for example by pressure), the process by which this reversal is brought about.
    2. Polysynthetic twinning ,
    repeated twinning of crystal lamellæ, as that of the triclinic feldspars.
    Repeated twinning ,
    twinning of more than two crystals, or parts of crystals.
    Twinning axis, Twinning plane .
    See the Note under Twin, n.