The rehaut is the vertical or slanted band that separates the dial from the inner edge of the case, beneath the crystal. It is not the bezel, it is not the dial: it is the cylindrical space between the two. Often overlooked, but from the workbench you can immediately tell how much work has gone into it.
Its primary function is structural: the rehaut acts as a shoulder for the crystal and visually seals the transition between the glass and the dial plane. In dive watches, like the Citizen Promaster Mechanical Diver, this band is pronounced and creates real optical depth between the crystal and the dial plane. In the VPC Type 37HW the rehaut has been minimized to lower the total case height to 9.8 mm without sacrificing 120 meters of water resistance: a deliberate design choice, not an aesthetic detail.
When the surface is inclined toward the dial, as in the Certina DS-X GMT or the Unimatic Modello Cinque, the effect is to visually enlarge the dial: the perceived diameter increases without touching the actual case dimensions. It is one of watchmaking's honest tricks.
Functional scales are often found on the rehaut. Seventies Yema watches used it systematically: the Rallygraf displayed the elapsed time scale, the Flygraf fuel conversion charts. The Nodus Obscura II prints ISO values and aperture settings there. The Doxa Sub 250T GMT features a 24-hour GMT scale in two colors, day and night. The Breitling Superocean places the minute scale in anthracite there. All examples where the rehaut works, not decorates.
Rolex introduced around 2004 laser engraving of its name repeated on the rehaut of nearly all its models. The alignment precision required makes this solution one of the hardest to replicate for counterfeiters: I still see this when pieces are brought to me for verification.
The finish can be brushed, guilloché with a diamond point like in the Polerouter SAS, or mirror polished. The TAG Heuer Carrera Glassbox even has a curved rehaut that follows the dome of the sapphire crystal, accentuating the three-dimensionality of the entire dial. Not all manufacturers choose to enhance this area, but when they do thoughtfully, the result substantially changes the perception of the watch.