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.
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