Short answer

Cartier does not publish a 21mm Clé de Cartier. The catalogue lists the Clé de Cartier in 31mm, 35mm, and 40mm cases. A listing that says "fake Clé de Cartier 21mm" is either quoting a measurement other than the case, or selling a watch that copies the key-shaped crown while matching no genuine reference. Measure the case before you pay, then compare the number to the 31mm model.

What the 21mm figure measures

Three numbers get mixed up in product titles.

  • Case diameter. A 21mm case sits below every Clé de Cartier size. Cartier's smallest Clé is 31mm.
  • Bracelet width at the end link. Large Clé cases run near 20mm to 22mm at the bracelet. A seller who measured the bracelet can land on 21mm.
  • Lug width on an unrelated ladies' watch. Many quartz watches with a rectangular crown use 21mm lugs, and the Clé name gets attached to the title.

Size reference

  • 31mm: smallest published Clé de Cartier case.
  • 35mm: mid size.
  • 40mm: largest, and the size most often copied.

If a seller sends caliper photos, the jaws should sit on the case flanks at the 3 and 9 positions with the crown clear of the jaws. A reading that includes the crown adds 2mm to 3mm.

Movement

Small cases limit movement choice. Quartz fits under 30mm with room to spare. An automatic movement needs a case near 36mm or more for a standard rotor and mainspring barrel. Ask for the movement name and a photo of the rotor. A 28,800 vph mechanical movement steps 8 times per second. A quartz movement steps once per second. Watch the second hand under a loupe and count.

Case and finish checks

  • Crown: rectangular and faceted, with a flat key bar. A round cabochon is the wrong shape.
  • Crystal: sapphire on genuine pieces. Water beads on sapphire. Mineral glass scratches at 5.5 Mohs.
  • Dial print: "CARTIER" in a serif face, "SWISS MADE" at 6 o'clock. Thick strokes and spelling errors are the fastest tells.
  • Caseback: reference and serial engraved into the metal, not printed on top. Replica serials repeat across units.
  • Weight: record the number on a kitchen scale and compare it with a steel watch you own at the same diameter. Plated brass and hollow cases run light.

Prices

Replica prices move with the seller and the movement. There is no stable market number for a 21mm Clé de Cartier, because no genuine 21mm reference exists to copy. A listing that quotes a genuine retail figure as its reference point is using that number as sales copy.

Before you pay

  1. Ask for the case diameter measured with calipers, with a ruler in the same photo.
  2. Ask for the movement name and a photo of the rotor.
  3. Ask if the crystal is sapphire.
  4. Ask for the return window in writing.

If the seller cannot answer items 1 through 3, the size in the title is marketing.