This was the thinnest watch movement made from 1925 through the 2000's.
Cal. 21 was designed to be the thinnest watch movement available, and it achieved this goal notably: It retained the record for 21 years. Even without the record, this 90 year old movement design continues to be produced.
A relatively simple movement, Cal. 21 is hand wound and has a power reserve of 42 hours. It is a 9 ligne (20.4 mm) diameter movement and just 1.75 mm thick. It has 18 jewels. The movement originally oscillated at 18,000 A/h but later versions were boosted to 21,600 A/h.
Because it was so thin and proven, F. Piguet had much success selling ebauche versions of their Cal. 21 to other manufacturers.
- Audemars Piguet 2003, which took the record for thinnest movement at 1.64 mm