The domed crystal is not a secondary aesthetic detail: it's a constructive choice that changes the optical geometry of the watch, the behavior of light on the dial and, in more extreme cases, even the total thickness of the case. I've seen dozens of pieces pass through my bench where simply replacing a flat glass with a dome completely changed how you read the dial, even with identical indices and hands.
The principle is simple: a convex surface curves light rays before they reach the eye, creating a sense of depth that a flat crystal cannot provide. This explains why old dive watches with plexiglass crystals seem to have more vivid dials than many modern ones: acrylic material lends itself to curvature naturally, and the result is an almost three-dimensional reading with ambient light. Today synthetic sapphire is worked with the same geometries, although milling and polishing a sapphire dome are expensive and technically demanding operations. A domed sapphire crystal with a 7-millimeter sag, like those fitted on certain independent tourbillons, can be worth thousands of euros on its own.
The double dome variant adds a second curvature on the inner side: the convex surface facing outward and the concave one facing the dial work together to amplify the three-dimensional effect and, with proper anti-reflective treatment, to reduce spurious reflections that a simple dome can generate at the edges. Audemars Piguet pushed this logic in the Code 11.59 with an asymmetric curve that varies from 6 o'clock to 12 o'clock, achieving an optical behavior difficult to describe in words and practically impossible to reproduce in a CGI render: every tilt of the wrist changes the distribution of reflections on the dial in a way that lighting software cannot anticipate.
There's a mechanical aspect often overlooked: a heavily domed crystal increases the declared thickness of the watch. In the Trilobe Une Folle Soirée, the 17.8 millimeters total include a pronounced sapphire dome; mentally remove it and the watch would be much thinner. It's not a flaw: it's a geometric consequence of the stylistic choice. You need to know this when evaluating thickness on a technical sheet.
On the dial front, the dome isn't just about the crystal. Ressence builds its own dials in titanium with a domed profile integrated into the rotating module: here the curvature is structural, not a glass over a flat surface. It's an extreme case, but useful for understanding that the optical principle of the dome can be applied to the dial itself, not just its protection.