Nakano's theoretical development emphasizes the effect of seismic disturbances at large distances from the source and develops equations of first motion in P and S for several types of source mechanisms. These equations are compared to the methods of approach of various investigators, and two mechanisms in particular are singled out: a single couple, which represents motion along a fault, and a double couple, which represents a compressive and tensile stress at right angles. Methods of transformation and projection permit the application of the theory for an infinite homogeneous earth to the heterogeneous earth. Possible uses of S phases are noted. Single observations of the first motion of S offer the possibility of resolving the ambiguity in fault-plane solutions from P alone in which the single couple is the mechanism assumed. Identification of the second nodal surfaces of SV and SH offers a criterion for deciding which mechanism, the single couple or the double couple, is operative in particular earthquakes. Further, simple relations involving the ratios SH/SV, P /SH, P /SV suggest other approaches to the problem of motion at the source of an earthquake. S phases, however, are to be used with great care.