The Datejust’s case, 31 mm, 36 mm or 41 mm in diameter and guaranteed waterproof to a depth of 100 meters (330 feet), is a paragon of robustness and reliability. The middle case is crafted from a solid block of Oystersteel, a particularly corrosion-resistant alloy, or 18 ct gold. The case back, edged with fine fluting, is hermetically screwed down with a special tool that grants only Rolex watchmakers access to the movement. The Twinlock winding crown, fitted with a double waterproofness system, screws down securely against the case. Completely waterproof, the Oyster case provides optimum protection for the Datejust’s movement.