  We propose a general mechanism of multiple-Q ordering with noncollinear and
noncoplanar spin textures in itinerant magnets. By analyzing the fourth-order
perturbation with respect to the spin-charge coupling, we find that the
instability toward multiple-Q ordering is caused by (d-2)-dimensional
connections of the Fermi surfaces in the extended Brillouin zone by the
multiple-Q wave vectors: zero(one)-dimensional point (line) connections in the
two(three)-dimensional systems. The instabilities are obtained as the "fixed
points" in the two-parameter flow diagram, irrespective of lattice structures.
The hidden Fermi surface instability provides a universal origin of
noncollinear and noncoplanar instabilities common to frustrated and
unfrustrated lattice systems.
