In 1956 and early 1957 the situation changed dramatically. Following a suggestion by T. D. Lee and C. N. Yang (1956) that parity conservation, or mirror symmetry, was violated in the weak interactions, which included β decay, a series of experiments by Wu, Ambler, Hayward, Hoppes, and Hudson (1957), by Garwin, Lederman, and Weinrich (1957), and by Friedman and Telegdi (1957) showed conclusively that this was the case. The discovery of parity nonconservation had serious implications for the previous analyses of β decay, suggested new experiments, and paved the way for a final decision concerning the mathematical form of the interaction in Fermi’s theory.