  The theory of the preroughening transition of an unreconstructed surface, and
the ensuing disordered flat (DOF) phase, is formulated in terms of interacting
steps. Finite terraces play a crucial role in the formulation. We start by
mapping the statistical mechanics of interacting (up and down) steps onto the
quantum mechanics of two species of one-dimensional hard-core bosons. The
effect of finite terraces translates into a number-non-conserving term in the
boson Hamiltonian, which does not allow a description in terms of fermions, but
leads to a two-chain spin problem. The Heisenberg spin-1 chain is recovered as
a special limiting case. The global phase diagram is rich. We find the DOF
phase is stabilized by short-range repulsions of like steps. On-site repulsion
of up-down steps is essential in producing a DOF phase, whereas an off-site
attraction between them is favorable but not required. Step-step correlation
functions and terrace width distributions can be directly calculated with this
method.
