Among　the　twinning　modes　in　hexagonal　close-packed　(HCP)　metals,　the　mechanism　for　{11　(2)　over　bar2}＜　11　(2)　over　bar(3)　over　bar　＞　mode　is　particularly　confusing　and　controversial.　In　the　literature　reports,　there　are　three　possible　second　invariant　planes,　i.e.　the　K-2　planes　for　{11　(2)　over　bar2}＜　11　(2)　over　bar(3)　over　bar　＞　twinning　mode:　{11(2)over　bar＞(4)　over　bar}　which　has　been　widely　accepted　and　corresponds　to　a　three-layer　zonal　twinning　dislocation;　{11　(2)　over　bar(2)　over　bar}　that　is　deemed　unfavorable;　and　(0002)　which　has　only　been　observed　in　atomistic　simulations　and　corresponds　to　a　single-layer　twinning　dislocation.　{11　(2)　over　bar(4)　over　bar}　was　predicted　by　classical　twinning　theory　and　the　experimentally　measured　magnitude　of　twinning　shear　s　in　titanium　and　zirconium　seemed　to　agree　well　with　the　prediction.　However,　{11　(2)　over　bar(4)　over　bar}　has　never　been　verified　in　simulations　which　show　that　(0002)　should　be　the　K-2　plane.　This　conflict　has　not　been　resolved　due　to　the　lack　of　experimental　observation　of　the　structure　of　twinning　dislocations.　In　this　work,　scanning　transmission　electron　microscopy　(STEM)　observations　are　conducted　to　resolve　the　twin　boundary　structure　in　deformed　pure　titanium　on　the　atomic　scale,　combined　with　atomistic　simulations.　Atomic　resolution　STEM　unambiguously　shows　that　the　twinning　dislocation　only　involves　a　single　twinning　plane　and　the　K-2　plane　is　(0002),　which　is　consistent　with　the　atomistic　simulations.　The　STEM　results　also　reveal　a　half-shear-half-shuffle　process　which　is　manifested　by　a　unique　twin　boundary　structure　generated　by　the　glide　of　single-layer　twinning　dislocations.　To　explain　these　results,　the　lattice　correspondences　of　all　three　K-2　planes　are　examined　in　great　detail.　In　particular,　shear　and　shuffle　required　in　the　lattice　transformations　are　analyzed　inside　the　framework　of　classical　theory.　These　analyses　explain　well　why　(0002)　is　the　more　favorable　K-2　plane　than　{11(2)over　bar＞(4)　over　bar}　and　{11　(2)　over　bar(2)　over　bar},　and　properly　resolve　the　conflict　between　the　prediction　of　the　classical　twinning　theory　and　the　simulation　results.　(C)　2021　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.