The 849 (and similar JLC 849RSQ) is an hand-winding movement. The JLC 849 is made up of 123 pieces and is 1.85 mm thick. It runs at 21600 vph, has 19 jewels, and a power reserve of 35 hours. It is closely related to the JLC 839, introduced in 1975, differing only by the addition of one jewel (at the lower second wheel pivot) and a redesign of some of the bridges.
The JLC 849 is used in the thinnest mechanical wristwatch currently available, the 4 mm Jaeger-LeCoultre Master Ultra Thin Jubilee.
The two versions of the JLC 849 differ greatly in layout and application. The JLC 849 is used in the round Master Ultra Thin line, while the JLC 849RSQ is a tonneau-shaped movement used in the Grande Reverso Ultra Thin SQ. The 849RSQ is also skeletonized and decorated; It has 128 pieces and is slightly thicker at 2.09 mm.
Jaeger-LeCoultre currently uses this calibre in 4 watches.