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.