A precessional motion of the spin of the electron that is a consequence of the special theory of relativity. After the electron spin was postulated in 1925 it was soon realized that the calculation of the fine structure of atomic spectra in terms of spin was incorrect by a factor of 2. However, in papers published in 1926 and 1927 Llewellyn Hilleth Thomas showed that when the spin precession is calculated relativistically there is a factor of 1/2, known as the Thomas factor, which enables the fine structure to be calculated correctly.