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Most of the present understanding of the S=1 quantum spin chains displayingthe Haldane gap is coming from the so-called valence-bond-solid Hamiltonianwhich has an exactly known ground state. We show that this point ischaracterized by the onset of short-range incommensurate spin correlations inthe one-parameter family of Hamiltonians with biquadratic coupling: H = sum_iS_i S_{i+1} +alpha sum_i ( S_i S_{i+1} )^2. This gives a physical meaning tothis special point. We establish precise values for the gaps, correlations, thestring order parameter, and identify the VBS point as a *disorder* point in thesense of classical statistical mechanics. It is a quantum remnant of theclassical transition between a ground state with long-range N\'eel order and aground state with incommensurate long-range order.Comment: plain TeX, 24 pages + 11 epsf figures from usual uufile

Onset of incommensurability at the valence-bond-solid point in theS=1 quantum spin chain