Back in the day just as now, watch manufacturers have been exited by the Idea of making movements as thin as possible. In the early 1950’s Audemars Piguet, Jaeger LeCoulter and Vacheron pulled together and developed the world’s thinnest movement. The result being a movement only 1.64mm thick. It has some unusual features like the barrel being suspended from the barrel bridge and the balance impulse pin being fitted directly onto the balance. In my eyes as a watch repairer the greatest achievement was making it extra fiddly to work on and very impressive.