In　situ　straining　in　a　high-resolution　transmission　electron　microscope　and　molecular　dynamics　simulations　reveal　a　new　deformation　twinning　mechanism　in　the　face-centered-cubic　structure.　A　twin　forms　via　the　simultaneous　and　cooperative　activation　of　different　Shockley　partial　dislocations　on　three　(1　1　1)　layers.　The　synchronized　slip　produces　a　zero　net　Burgers　vector;　such　twining　relieves　local　stress　concentration　in　a　shear　confined　to　adjacent　atomic　layers,　but　induces　no　macroscopic　shape　change　of　the　surrounding　crystal.　(C)　2011　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.