Created in 1970, Calibre 2121 is a derivative of Calibre 2120 (1967), also powering an aperture-type date display. This additional function raised its thickness to 3.05 mm, making it the world's thinnest mechanical selfwinding movement with a central rotor and date.

The history of Calibres 2120 and 2121 will be told in detail in a dedicated article available soon.

Calibre 2121 was introduced in October 1970 in Model 5381. It was fitted in numerous ultra-thin selfwinding watches before being chosen in 1972 for the first Royal Oak, Model 5402. From that date onwards and until its replacement in 2022 by the 7121 (Model 16202), Calibre 2121 was one of the essential attributes of the Royal Oak "Jumbo" watches.

For a long time, Calibre 2121 blanks were produced in Le Sentier by LeCoultre & Cie (renamed Jaeger-LeCoultre in 1979). From the start of the 21^st century, it was produced by the Audemars Piguet workshops in Le Brassus.

During its first quarter-century of existence, nearly 20,000 Calibre 2121 watches were produced and used in numerous references across several collections. Since the dawn of the 21^st century, with a few exceptions, the calibre has been reserved for the Royal Oak “Jumbo”. Production between 2000 and 2021 exceeded 12,000 units. In 2012, an openworked 2121SQ variation was created for a model celebrating the 40^th anniversary of this ultra-thin collection. In 2022, the movement was replaced by Calibre 7121.