  Molecular-dynamics simulations of KNbO$_3$ reveal preformed dynamic
chain-like structures, present even in the paraelectric phase, that are related
to the softening of phonon branches over large regions of the Brillouin zone.
The phase sequence of ferroelectric transitions is correctly reproduced,
showing that the first-principles effective Hamiltonian used in the simulations
captures the essential behavior of the microscopic fluctuations driving the
transitions. Real space chains provide a framework for understanding both the
displacive and order-disorder characteristics of these phase transitions.
