Measurements of internal strain development during in-situ deformation of a textured magnesium alloy plate represent the first direct observations of intergranular (type II) strains, which allow for definitive statements about the magnitude of stresses within (i) parent grains when mechanical twins are beginning to form, (ii) the mechanical twins themselves as they are being formed, and (iii) the development of stresses within twins during subsequent deformation. These data will be extremely valuable for developing accurate constitutive models incorporating twinning.