The quantities characterizing wave propagation through the interface of two media or through a transmission line containing obstacles are the two reflection coefficients r1 and r2 and the two transmission coefficients t1 and t2. It is shown that for any lossless discontinuity |r1| = |r2|, argt1 = argt2, and that argr1+argr2 −2 argt1 = ±π. 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.