twinning

Twinning refers to the phenomenon of two or more offspring produced by a single pregnancy, or more broadly to the process by which two or more structures share a common origin or orientation. In biology, it is most commonly discussed in humans and other mammals; in materials science, crystallography uses the term to describe intergrown crystal domains; in geography or cultural studies, twinning relates to the pairing of towns or institutions.

Biological twinning is categorized by zygosity. Monozygotic twins arise from one fertilized egg that splits, producing genetically identical individuals; dizygotic twins come from two separate eggs, sharing about 50 percent of genes, like any siblings. The timing of splitting determines placentation: dichorionic-diamniotic if splitting occurs early; monochorionic-diamniotic or monochorionic-monoamniotic if later; rare conjoined twins result from very late separation. Twin pregnancies carry higher risks, such as preterm birth and twin-to-twin transfusion syndrome in monochorionic placentation.

Crystallographic twinning occurs when two or more crystal domains share lattice orientation related by symmetry operations not in the crystal's single lattice. Twins can form during growth or through post-growth stress. Twin laws classify the relative orientation; twinning affects crystal symmetry, intensities in diffraction experiments, and material properties.

In a sociocultural context, town twinning or sister city programs create formal links between communities, often for cultural exchange, tourism, and economic collaboration. The practice grew after World War II to promote peace and understanding, with thousands of pairs of twin towns worldwide.