  We investigate the physical properties of two coupled chains of electrons,
with a nearly half-filled band, as a function of the interchain hopping t_\perp
and the doping. We show that upon doping, the system undergoes a
metal-insulator transition well described by a commensurate-incommensurate
transition. By using bosonization and renormalization we determine the full
phase diagram of the system, and the physical quantities such as the charge
gap. In the commensurate phase two different regions, for which the interchain
hopping is relevant and irrelevant exist, leading to a
confinement-deconfinement crossover in this phase. A minimum of the charge gap
is observed for values of t_\perp close to this crossover. At large t_\perp the
region of the commensurate phase is enhanced, compared to a single chain. At
the metal-insulator transition the Luttinger parameter takes the universal
value K_\rho^*=1, in agreement with previous results on special limits of this
model.
