Studies show that there is a genetic proclivity for dizygotic twinning. The predisposing factors of monozygotic twinning are unknown. Crystal twinning occurs as simple contact twins. Tallinn participates in international town twinning schemes to foster good international relations. Penetration twinning on the crystallographic direction, {111}. One mechanism that aids in this process is the twinning of the martensite phase. Cyclic twins are caused by repeated twinning around a rotation axis. Where known, the date of formation of the twinning agreement is included in parentheses. Twinning and slip are competitive mechanisms for crystal deformation. Below M, martensite formation under load allows superelasticity due to twinning. The reverse process however is not entirely homogeneous and crystal twinning occurs. The twinning between some of the former communes, merged with Bruges in 1971, were discontinued. The geometry of the twinning is controlled by the mineral's symmetry. Sapporo has twinning relationships with several cities worldwide. It is organised by the Chesham Town Twinning Association.