DISC

In general, this refers to a flat, solid cylinder of small thickness, most often able to rotate about its central axis. Many types of discs are used in tooling (polishing discs, indexing discs, etc.). In watch movements, discs are most commonly components of alphanumeric displays (such as day or month discs in a perpetual calendar). The values to be displayed are printed on the disc and appear through an aperture in the dial. It is worth noting that people commonly speak of a “date disc,” even though it is most often actually a ring.