  Lars Onsager announced in 1949 that he and Bruria Kaufman had proved a simple
formula for the spontaneous magnetization of the square-lattice Ising model,
but did not publish their derivation. It was three years later when C. N. Yang
published a derivation in Physical Review. In 1971 Onsager gave some clues to
his and Kaufman's method, and there are copies of their correspondence in 1950
now available on the Web and elsewhere. Here we review how the calculation
appears to have developed, and add a copy of a draft paper, almost certainly by
Onsager and Kaufman, that obtains the result.
