In deformation twinning, twin boundaries (TBs) should coincide with the twinning plane. Here we show that the TBs of the most common twinning mode in hexagonal close-packed metals, f1 0 1 2gh1 0 1 1i, may not lie on the f1 0 1 2g twinning plane. Examinations using transmission electron microscopy (TEM) reveal that the TBs in Co and Mg deviate significantly from the f1 0 1 2g plane. High-resolution TEM confirms that the incoherent TBs entirely depart from the twinning plane with a magnitude greater than 45°. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.Keywords: Twin; Twin boundary; Deformation twinning; Cobalt; Magnesium When crystalline solids are subjected to plastic deformation, in addition to dislocation slip, deformation twinning can be activated to accommodate the microscopic strain. A deformation twin, which predated the concept of dislocation (1934), was defined as "a region of a crystalline body which has undergone a homogeneous shape deformation in such a way that the crystal structure of the resulting product is identical with that of the parent, but oriented differently" [1]. Because the two structures are identical, the boundary plane, also known as the twinning plane, must remain invariant during twinning. The parent and the twin lattices are reflected about the twinning plane.The homogeneous shear involved in twinning carries parent atoms to the twin lattice [2]. To accomplish a homogeneous shear on a twinning plane, passage of twinning dislocations at the interface is required to complete such displacements. A twinning system comprising a twinning plane and a twinning direction along which the shear takes place can be rigorously defined in this scenario. The twin boundary (TB), i.e. the interface between parent and twin, has to coincide with the twinning plane, although microscopically local disregistry is permissible in the presence of twinning dislocation loops. In face-centered cubic (fcc) metals such as aluminum, copper, nickel, etc., the twinning plane is identical to the slip plane of dislocations, i.e. the close-packed {1 1 1} planes, and the twinning dislocations are Shockley partials 1 6 h1 2 1i. In low-symmetry, hexagonal close-packed (hcp) materials such as Mg, which have attracted tremendous attention in the quest for lightweight vehicle design, twinning plays a crucial role in plastic deformation and strain-path anisotropy [3]. On one hand, the easy slip directions contained in the basal plane are incapable of accommodating strain when the unit cell is loaded normal to the basal plane. On the other hand, non-basal slip systems are harder to activate than twinning by at least a factor of 3. The most frequently observed twinning mode in all hcp metals, which is always believed to be on the f1 0 1 2g plane and parallel to the h1 0 1 1 i direction, predominates in plastic deformation among multiple twinning modes. Within the framework of classical deformation theories, numerous crystallographic models have attempted to resolve twinning dislocations on the tw...