Composition plane
From Online Dictionary of Crystallography
Plan d'accollement, surface d'accollement (Fr). Verwachsungsebene (Ge). Piano di composizione, superficie di composizione (It). 接触面 (Ja). Plano de composición (Sp).
The surface that separates two individuals (domains) in a twin is called composition surface or, when it has the regular shape of a plane, composition plane. The former is characteristic of penetration twins, the latter of contact twins.
In the majority of twins the composition plane is parallel to the twin plane (for reflection twins) or normal to the twin axis (for rotation twins) but exceptions do exist.