Rolex developed and patented the screw-down crown for their Rolex Oyster case in 1926. This gave the company an early lead in developing reliable dive watches, a market the company dominates to this day.
Twinlock refers to a screw-down crown with extra water resistance. It is identified by a marking on the crown: Two dots on gold models, one dot on platinum, or a horizontal line on steel.
Triplock is an enhanced system for professional dive watches. It enables depth ratings over 100 meters. Triplock is identified by three dots below the Rolex emblem on the watch crown.