In The Theory of Sound, Rayleigh presents his analysis of the reciprocal theorem of mathematical physics with the observation that “Very remarkable reciprocal relations exist between the forces and motions of different types….” in a linearized dynamical system. He introduces the reciprocity principle for sound waves in an air‐filled space: “On his extension of Green's theorem, Helmholtz founds his proof of the important (reciprocal) theorem contained in the following statement: ….” In the ensuing 100 years since publication of his book, the reciprocity principle has been extended to many areas of physics—electrodynamics, electroacoustics, piezoelectricity, electromagnetic waves, optics, etc. The principle has been successfully applied in electroacoustics to achieve a system for accurate, absolute calibration of microphones used for acoustical measurements. The system is now internationally used by standards laboratories. Limitations and restrictions on applicability of the reciprocity principle to certain physical systems have come to light. Conspicuous examples involve the propagation of sound in moving air and the propagation of electromagnetic waves through magnetized media.