  Exact and rigorous solutions of the ground-state problem for the classical
Heisenberg model with nearest-neighbor interactions on two- and
three-dimensional lattices composed of zigzag (triangular) ladders are obtained
in a very simple way, with the use of a cluster method. It is shown how the
geometrical frustration due to the presence of triangles as structural units
leads to the emergence of incommensurate spiral orderings and their collinear
limits. Interestingly, these orderings are determined by a single angle (along
with the signs of the interactions between neighboring spins); therefore, the
term "single-angle spiral orderings" is proposed.
