The low energy dynamics of the anti-ferromagnetic Heisenberg spin S chain in the semiclassical limit S → ∞ is known to map onto the O(3) nonlinear σ model with a θ term in 1+1 dimension. Guided by the underlying dual symmetry of the spin chain, as well as the recently established topological significance of “dangling edge spins,” we report an exact mapping onto the O(3) model that avoids the co...