  Dihedral codes, particular cases of quasi-cyclic codes, have a nice algebraic
structure which allows to store them efficiently. In this paper, we investigate
it and prove some lower bounds on their dimension and minimum distance, in
analogy with the theory of BCH codes. This allows us to construct dihedral
codes with prescribed minimum distance. In the binary case, we present some
examples of optimal dihedral codes obtained by this construction.
