ABSTRACTThe transmitter and receiver coils and the line joining them in an electromagnetic moving source‐receiver system can, in principle, have an infinity of mutual orientations. Let the direction of a survey line (traverse direction) bexand let the vertical plane through it be thex‐zplane and let thez‐axis be a line in this plane perpendicular to thez‐axis, then the y‐axis of an orthogonal coordinate system is automatically determined. A definite coordinate system, dependent on the mutual coil orientation alone, and independent of the spatial direction of the survey line or alterations in it, is thus associated with each point. The geometry of a moving source‐receiver configuration is uniquely defined by specifying (i) the direction cosinesa, b, cof the transmitter coil axis T, (2) the direction cosinesd, e, fof the line L joining the transmitter and the receiver coils and (3) the direction cosinesg, h, iof the receiver coil axis R. Hence a set of indices conveniently written as Ta, b, cLd, e, fRg, h, idescribes the geometry and also enables “reconstruction” of the system uniquely and without the risk of confusion inherent in such incomplete descriptions as “vertical coil system”, “horizontal coil system” etc. Degeneracy arises if the traverse direction is vertical. How this is overcome is described in the paper. To the above indices may be appended the indices T, the distance between T and R and v, the frequency.For example, the most common “horizontal coplanar coil system” has the indices T 001 L 100 R 001. An airborne wing‐tip system with coil axes in the flight direction is specified by T 100 L 010 R 100. Other examples are given in the paper. Maps and data sheets of moving source‐receiver systems should be marked with