  This paper presents a new approach to studying galactic structures. They are
considered as the low-frequency normal modes in a disc of orbits precessing at
different angular speeds. Such a concept is an adequate alternative to the
commonly used approach of treating the disc as a set of individual stars
rotating at near-circular orbits around the centre. The problem of determining
the normal modes is reduced to a simple integral equation in the form of the
classical eigen-value problem, where the eigen-value is directly equal to the
pattern speed of the mode, $\Omega_p$. An examination of the general properties
of the basic integral equation shows that two types of solutions exist,
bar-like and spiral. The numerical solutions of both types are obtained. The
characteristic pattern speeds are of the order of the mean orbit precession
speed, although for the bar-modes $\Omega_p$ can markedly exceed the maximum
precessing speed of orbits. It is shown that the bar-mode grows due to the
immediate action of its gravitational field on the stars at the resonance
regions. As for the spiral mode, its excitation is probably due to the inner
Lindblad resonance that can promote mode growth.
