To avoid the loss of information in the coarser and the smaller fraction, results of size analysis of sample can be plotted in three-component diagrams, also called ternary diagrams or triangular diagrams, the sample being represented by a single point. A particular disposition of triangular diagrams called a sequence of triangular diagrams can be formed by considering the alpha grade scale (10 v 10) as the successive boundaries of each diagram. In this system, a sample is represented by a curve which is a logarithmical spiral , the equation being: rho = a e (super -Ktheta ') . Geometric and trigonometric relationships give rho and theta '. Study of the variations of the K coefficient permits one to obtain the mathematical position of truncation points of log-probability curves. In this graphical representation of samples, the straight lines, which join truncation points, correspond to sedimentary populations (Visher, 1969). A spiral curve is formed by several arches of which we know the equations. These arches are homologues of the straight lines of sedimentary populations in log-probability curves. The logarithmic spiral gives: first, an equation for the size analysis of sediments, and secondly, greater accuracy in determination and study of sedimentary populations which depend on the different modes of transport and deposit of sediment.