Split-seconds chronograph wristwatches were the "must" of Patek Philippe technology and after 1938 were generally cased with this reference. The majority are in yellow gold. Rarely they are in pink gold and only four stainless steel examples are known three of which were sold by Antiquorum: Geneva April 10 1994 lot 431 Geneva April 23 1995 lot 457 Geneva May 10 2009 lot 161. In the mid-1950s some were produced with a coaxial button on the winding crown for the split-seconds functions (stop and reunite). Production of this reference ceased in 1971. First generation: stop and reunite functions of the split-second hand controlled by the winding crown. Second generation: stop and reunite functions of the split-second hand controlled by a co-axial button in the winding crown. The Split-Seconds Chronograph was designed to time two events which begin simultaneously but conclude at different times as well as a single event for which an intermediate timing is necessary such as horse or car races. Patek Philippe was one of the fi rst to introduce modern split-seconds chronographs - as early as 1862. (No. 17557 see Antiquorum October 18 1992 lot 590.) The split-seconds mechanism employs two central chronograph hands. Both hands are started at the same time. The splitseconds hand can be stopped while the chronograph hand continues to move. The split-seconds hand can be reunited with the chronograph hand in order to time another event. This complication is especially useful during sporting events such as a horse race a car race or a ski race.