  We analyze the phase diagram of a system of spin-1/2 Heisenberg
antiferromagnetic chains interacting through a zig-zag coupling, also called
zig-zag ladders. Using bosonization techniques we study how a spin-gap or more
generally plateaux in magnetization curves arise in different situations. While
for coupled XXZ chains, one has to deal with a recently discovered chiral
perturbation, the coupling term which is present for normal ladders is restored
by an external magnetic field, dimerization or the presence of charge carriers.
We then proceed with a numerical investigation of the phase diagram of two
coupled Heisenberg chains in the presence of a magnetic field. Unusual
behaviour is found for ferromagnetic coupled antiferromagnetic chains. Finally,
for three (and more) legs one can choose different inequivalent types of
coupling between the chains. We find that the three-leg ladder can exhibit a
spin-gap and/or non-trivial plateaux in the magnetization curve whose
appearance strongly depends on the choice of coupling.
