Two reciprocal theorems, the generalized Rayleigh theorem and the Sommerfeld-Pfrang theorem are of great theoretical importance in radiocommunication. A careful analysis of these theorems and their mathematical derivations shows that they are quite distinct and their practical fields of application different. In particular it shows that the Sommerfeld-Pfrang theorem labors under restrictions, implicit in its mathematical derivation, which seriously limit its field of practical applicability.