Atomistic　simulations　were　conducted　to　study　the　migration　of　{1012}　⟨1011⟩　twin　boundary.　A　bi-crystal　that　satisfies　the　twin　orientation　relationship　was　constructed　and　a　tensile　strain　was　applied　in　parallel　to　the　⟨c⟩　axis　of　one　crystal.　Under　the　tensile　strain,　the　twin　boundary　starts　to　migrate　but　the　migration　(twin　growth)　does　not　produce　any　observable　shear　strain　on　the　bi-crystal.　In-situ　transmission　electron　microscopy　(TEM)　observations　of　a　single　crystal　Mg　under　tension　and　compression　confirm　that　during　twinning　and　detwinning,　no　shear　strain　is　produced.　The　specimen　uniformly　elongates　and　narrows　during　twinning,　and　widens　across　the　width　during　detwinning.