Screw Down Crown
The crown is used for winding and setting a watch, a screwed down one is locked by being screwed into the watch’s case.
Having it screwed in greatly helps prevent dust and water entering into the watch. Because of this advantage it is widely used on professional diving watches.
The founder of Rolex, Hans Wilsdorf invented the screw crown and filed a patent for it in 1926. The next year Rolex introduced their first waterproof watch called the Rolex Oyster and the same year Mercedes Gleitze publicized its waterproof qualities when she wore it for her 15 hour swim of the English channel.