The key to the success of the Oyster case in preventing water from entering the interior of the watch is the safety of its upper crown. The crown has traditionally been the “Achilles heel” of watch design, a place where moisture and other junk can easily get in, and you don’t want any slight movement.
Early examples of the Oyster case featured a patented screw-in winding crown mechanism with a metal gasket that was held securely in the tube of the central case, creating a watertight seal. When the first Submariner was produced in 1953, its system had to be braced to withstand the extra pressure, so Rolex designed the Twinlock crown with a second gasket system inside the tube to create two sealed areas. O-rings are also made of synthetic material to create a stronger barrier.

In 1970, with the introduction of the Sea-Dweller, Rolex again strengthened the winding crown and introduced the Triplock, a new arrangement of gaskets that created three sealed areas to keep moisture out. This time, two are in the case tube and the third stays under the crown.
As you can imagine, three of Rolex’s professional diver’s watches feature triple-lock crowns to ensure their incredible water resistance, but so do some other watches in the catalog. The watches with an * star in the list above (the two Yacht-Masters, the GMT-Master II and the Daytona) are equipped with Triple Crowns, but still only reach the 100m class. In their case, the decision to include the three-ringed crown may have been partly an aesthetic choice. The triple crown is larger than the double lock and is in greater proportion to the rest of the crown.