Quick answer

Cartier does not make a 30 mm Clé de Cartier. The model launched in 2015 in three case sizes: 31 mm, 35 mm, and 40 mm. Sellers who list a replica as 30 mm are describing the 31 mm small case, measured without the crown or rounded down in the listing title. Measure the case across 3 and 9 o'clock with a caliper. A true 31 mm case reads 30.8 mm to 31.2 mm. A case that reads 30.0 mm came from a different drawing.

Case sizes and strap widths

Case size drives everything else on this watch. The lugs get wider as the case grows. Measure the gap between the lugs. That number tells you which case you have, even when the listing title is wrong.

  • 31 mm case: lug gap near 16 mm
  • 35 mm case: lug gap near 18 mm
  • 40 mm case: lug gap near 20 mm

Ask the seller for a photo of the caliper across the lugs and across the case. Two photos settle the question.

The key-shaped crown

The Clé de Cartier takes its name from the crown. It is shaped like a key and capped with a sapphire or spinel cabochon. On the genuine watch the key turns to wind the movement and pulls out to set the time. On replicas the crown often does one job and not the other. Test the turn. A crown that spins with no resistance does not connect to the movement. Test the pull. A crown that pulls out with a grind will strip.

Movement check

Watch the second hand. A quartz movement jumps one step per second. A mechanical movement sweeps in five to eight small steps per second. The 31 mm copies ship with quartz more often than the 35 mm and 40 mm copies. Cartier fits automatic movements in most 35 mm and 40 mm references. Confirm the movement type before you pay. Caseback engraving is not proof.

Dial details to compare

  • Roman numerals: check the X and the VII for even stroke weight
  • Hands: blued steel sword shape, no burrs at the tip
  • Date window: sits at 3 o'clock, frame edges cut square
  • Printing: the "Cartier" signature sits under the 12, no ink bleed

Thickness

Replica cases run thicker than the Cartier original in most samples. Thickness figures for the genuine watch are published per reference by Cartier. Measure the replica from caseback to crystal with the same caliper. A gap of 1 mm or more against the factory number points to a case built for a generic movement.