Llewellyn Thomas, a British physicist on a research visit to Niels Bohr's Institute in Copenhagen, solves within a few days the problem that threatened to sink the newly proposed electron spin hypothesis of Uhlenbeck and Goudsmit: the theory predicted a doublet splitting in the anomalous Zeeman effect exactly twice the observed experimental value. Thomas realizes the problem is not physical but purely kinematic: when an electron moves along a curved (non-rectilinear) orbit at relativistic speeds, two successive, non-collinear Lorentz transformations are not equivalent to a single Lorentz transformation, but to a Lorentz transformation combined with an additional spatial rotation. That rotation — today called Thomas precession — reduces the classically expected spin-orbit coupling by exactly half, and once incorporated, the electron spin theory agrees perfectly with experiment. Thomas, staying at Bohr's house over a Christmas weekend, presents the result to Bohr and Kramers, who insist on sending it immediately to Nature, where it appears in April 1926. The kinematic effect itself had already been noted, with no physical application, by several mathematicians in 1913 (Borel, Föppl and Daniell, Silberstein) and almost simultaneously by Yakov Frenkel in 1926; but it is Thomas's concrete physical resolution that saves the spin hypothesis and turns it into an indispensable calculation tool, standard today in particle accelerator physics (polarized-spin storage rings) and in the Bargmann-Michel-Telegdi equation (1959) itself, which relativistically generalizes Thomas precession for particles with anomalous magnetic moment.