In watchmaking, the term seal covers two distinct realities that should never be confused: the hermetic sealing of the case and the qualitative certification of the movement. Confusing them is a common mistake, even among sellers.
On the sealing front, the seal is the set of systems that prevent water, moisture, dust, and unwanted gases from entering the case. The most widespread solution is the O-ring gasket — positioned at the crown, caseback, and crystal. Mido was among the first to industrialize the concept with the Aquadura system of 1930: a cork gasket on the crown, primitive but effective for the time. In the 1950s, EPSA patented the Super-Compressor system, where external water pressure progressively tightened the seal rather than fighting it: an elegant mechanical intuition, used on IWC Aquatimer Ref. 812 and 816. The screw-down crown works differently: it mechanically blocks access to the movement by rotating on threads, and is today the standard on professional diving watches. The ZRC Securicode pushed the concept further with a double-chamber caseback, where the gasket sealed independently of the threading. Seiko, for the 6159-7010 at 600 meters without a vent valve, developed an L-section gasket that created a double barrier against helium during saturation diving. Montblanc adopted a different approach with the Zero Oxygen system: a core that removes oxygen between crystal and case, preventing condensation and oxidation during extreme temperature swings. Omega's NAIAD system instead acts on the orientation of the sapphire caseback, ensuring the gasket compresses uniformly with each closure.
On the certification front, the Poinçon de Genève — commonly called the Geneva Seal — is a quality mark that attests to precise standards of movement finishing and construction: Genève stripes, anglage, perlage, and so on. Since 2016, Vacheron Constantin applies it to all Overseas. It does not certify chronometric precision in the strict sense, but manufacturing quality. It is awarded only to movements produced and finished in the Canton of Geneva.
A third use of the term is purely qualitative-commercial: the vermillion seal of Hajime Asaoka on every Kurono Tokyo dial is a stamp of personal approval, not a third-party certification. Different in nature, but equally binding in the production process.
From the bench I will add that the most neglected seal is often the pressure caseback on vintage watches from the 1960s-1970s: the gasket hardens, loses elasticity, and no one replaces it until it's too late. Before any contact with water, on any watch, I always verify the condition of the gaskets. It is the least glamorous and most important check.