A transparent case back (often sapphire crystal) that shows the movement.
Extended Explanation:
Usually made of sapphire crystal, an exhibition case back allows wearers to admire the craftsmanship of the movement. Luxury brands often decorate their movements with perlage, Côtes de Genève, or hand-engraving. Collectors love this feature, as it transforms the watch into both a timekeeper and a miniature piece of mechanical art.
