  Twinning is an important deformation mode of hexagonal close-packed metals.
The crystallographic theory is based on the 150-years old concept of simple
shear. The habit plane of the twin is the shear plane, it is invariant. Here we
present Electron BackScatter Diffraction observations and crystallographic
analysis of a millimeter size twin in a magnesium single crystal whose straight
habit plane, unambiguously determined both the parent crystal and in its twin,
is not an invariant plane. This experimental evidence demonstrates that
macroscopic deformation twinning can be obtained by a mechanism that is not a
simple shear. Beside, 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 a new theory for the displacive
transformations
