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Nucleation and propagation of tensile twins in magnesium alloy AZ31 are investigated for a large number of twins at an early stage of their development. High-resolution electron backscatter diffraction (HREBSD) techniques are employed to give additional insights. Correlations with grain orientation, boundary misorientation and active slip systems are observed in the region of twins that arise at grain boundaries. Two types of twin are identified: (1) slip-assisted twins that nucleate at grain boundaries with no apparent influence from nearby twins, and (2) twin-assisted twins that result from twins propagating across a grain boundary. Twinning occurs in “hard” grains that cannot accommodate necessary contraction via -type slip. Slip assisted twins nucleate at high-angle boundaries. Twin-assisted twinning occurs at low-angle boundaries. The distributions of grain boundary misorientation associated with each type of …