The quantities characterizing wave propagation through the interface of two media or through a transmission line containing obstacles are the two reflection coefficientsr1andr2and the two transmission coefficientst1andt2. It is shown that for any lossless discontinuity |r1| = |r2|, argt1= argt2, and that argr1+argr2−2 argt1= ±&pgr;. These relations are applied to the calculation of reflections from sheets of dielectric and metal‐plate media, the latter being distinguished by the fact that argt1≠0.