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Twinning is an important deformation mode of hexagonal close-packed metals. The crystallographic theory is based on the 150-year-old concept of simple shear. The habit plane of the twin is the shear plane; it is invariant. This article presents electron backscatter diffraction observations and crystallographic analysis of a millimetre-size twin in a magnesium single crystal whose habit plane, unambiguously determined both in the parent crystal and in its twin, is not an invariant plane. This experimental evidence demonstrates that macroscopic deformation twinning can be achieved by a mechanism that is not a simple shear. This unconventional twin is often co-formed with a new conventional twin that exhibits the lowest shear magnitude ever reported in metals. The existence of unconventional twinning introduces a shift of paradigm and calls for the development of new crystallographic theories of displacive transformations.

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