Determination of the Spin Axis in Quantum Spin Hall Insulator Candidate Monolayer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>WTe</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Evidence for the quantum spin Hall (QSH) effect has been reported in several experimental systems form of approximately quantized edge conductance. However, most fundamental feature QSH effect, spin-momentum locking channels, never demonstrated experimentally. Here, we report clear evidence channels monolayer WTe2, thought to be a two-dimensional topological insulator (2D TI). We observe that conductance is controlled by component an applied magnetic field perpendicular particular axis, which identify as axis. The axis same all edges, situated mirror plane tungsten chains at 40$\pm$2{\deg} layer normal, implying spin-orbit coupling inherited from bulk band structure. show this finding consistent with theory if band-edge orbitals are taken have like parity. conclude parity assignment correct and both states bands WTe2 share simple Combined other known features establishes locking, therefore truly natural 2D TI.