  A cold gas of particles with anisotropic interactions of general form, due to
a polarizing field, is studied. Special cases are atoms or molecules with
dipole-dipole or quadrupole-quadrupole interactions. It is shown that the
angular dependence of an observable on the direction of the polarizing field is
largely determined by symmetry. For a gas in a confined quasi two-dimensional
geometry, the effective interaction is calculated in general form. Some
examples of dipole and quadrupole gases are considered. It is concluded that
when anisotropic forces are studied in a general manner, one can obtain simpler
results and better understanding for some problems.
