  In quantum spin chains, it has been observed that the incommensurability
occurs near valence-bond-solid (VBS)-type solvable points, and the correlation
length becomes shortest at VBS-type points. Besides, the correlation function
decays purely exponentially at VBS-type points, in contrast with the
two-dimensional (2D) Ornstein-Zernicke type behavior in the other region with
an excitation gap. We propose a mechanism to explain the onset of the
incommensurability and the shortest correlation length at VBS-like points. This
theory can be applicable for more general cases.
