The JLC 920 is an ultra-thin automatic calibre developed and produced by Jaeger-LeCoultre.
Cal. 920 is famous as the only calibre to be used by all three top watch brands, Patek Philippe, Vacheron Constantin and Audemars Piguet, and no others. Manufactured only as an ebauche movement by Jaeger-LeCoultre, Cal. 920 was sold as the Audemars Piguet 2120/2121/2122, Patek Philippe 28-255, and Vacheron Constantin 1120/1121/1122. Although manufactured by Jaeger-LeCoultre, the 920 was never used in its own-brand watches. The rights to the design are said to have passed to Vacheron Constantin when Jaeger-LeCoultre was acquired by the Richemont Group in 2000, and their calibre 1120 remains in production.
The JLC 920 and derivatives are extremely thin, measuring just 2.45 mm with a full-sized central rotor for winding. Note that, contrary to some commenters, the Patek Philippe 28-255 did not use a micro rotor. A date module brings thickness to 3.05 mm, while date and calendar modules bring thickness to 3.40 mm. Diameter is fairly standard at 12½ ligne (26.00 mm).
Cal. 920 was introduced in 1967 and continues in production today, with Audemars Piguet and Vacheron Constantin still selling the movement. Patek Philippe replaced their version, Cal. 28-255, with their in-house PP 335 and PP 310 in the 1980's and no longer uses this movement though it took them a decade to perfect their in-house replacement. Although it is a fairly old design, it remains one of the thinnest automatic movements on the market even today, only edged out by the Piaget 1205P by 0.05 mm.
The 920 features 36 jewels and Patek Philippe's famed free-sprung Gyromax balance. Four ruby wheels support the full-diameter rotor, which runs on a beryllium rail for stability.