  Most of the present understanding of the S=1 quantum spin chains displaying
the Haldane gap is coming from the so-called valence-bond-solid Hamiltonian
which has an exactly known ground state. We show that this point is
characterized by the onset of short-range incommensurate spin correlations in
the one-parameter family of Hamiltonians with biquadratic coupling: H = sum_i
S_i S_{i+1} +alpha sum_i ( S_i S_{i+1} )^2. This gives a physical meaning to
this special point. We establish precise values for the gaps, correlations, the
string order parameter, and identify the VBS point as a *disorder* point in the
sense of classical statistical mechanics. It is a quantum remnant of the
classical transition between a ground state with long-range N\'eel order and a
ground state with incommensurate long-range order.
