Lamellae of β-tin with 〈010〉 normals have been found to be suitable specimens for an electron microscopic study of twinning. The lamellae, formed by cellular precipitation in a lead-tin (5.5 at. %) alloy at room temperature, were isolated by chemical dissolution of the lead matrix. Twinning dislocations were observed on the incoherent boundary of a (301) deformation twin during its propagation in the lamella. The twinning dislocations tended to group together to appear as segments which had an average step height of 24 A and a (103) Burgers vector of 2.4 Å. Under the beam action, the deformation twins were observed to nucleate heterogeneously from certain grain and twin boundaries, and to grow with a speed of about 200 Å/see and 3 Å/see in the longitudinal and normal direction respectively. The lamellae exhibited both growth twins and deformation twins. Both have 〈301〉 and (1̄01) as the twinning plane. The (301) and (1̄01) twins are reciprocals of each other. It is shown that when a twin intersects its reciprocal, a tilt type rotational relation exists. The equilibrium morphology of one such intersection has been studied to estimate the torque needed for rotating the coherent twin boundary. © 1970.