  We show that lattice mismatched bilayer graphene can realize robust
one-dimensional wires. By considering a single domain wall where the masses of
the Dirac electrons change their sign, we establish a general projection
principle. This determines how the existence of topological zero-energy domain
wall states depends on the direction of the domain wall and locations of the
massive Dirac cones inside the bulk Brillouin zone. We generalize this idea for
arbitrary patterns of domain walls, showing that the topologically protected
states exist only in the presence of an odd number of topological domain walls.
