The short answer

The 31mm figure points to the Tank Must XL, the largest steel Tank in Cartier's current line. It runs about 41mm from lug to lug and 31mm across the case, which keeps it in dress-watch territory while still reading as a modern watch rather than a vintage piece. On a wrist under roughly 6.5 inches it feels big. If you normally wear 38 to 40mm round watches, it lands comfortably. Replicas of this size are everywhere because a rectangle case with a printed dial is far easier to copy than a complicated round one, so factories can get close on looks without matching the original's construction.

How the 31mm rectangle actually wears

A rectangle sits on the wrist differently than a circle. The Tank Must XL covers more of the wrist top than a 31mm round watch, because the case runs lengthwise along the arm. Lugs are short and tucked, so the strap drops down steeply and the watch hugs rather than floats. Thickness matters more than width here. The genuine automatic sits under 9mm, and a copy that measures 11 or 12mm loses the entire point of the design. If you only check one spec before buying, check the thickness.

The caseback curve is the other thing to eyeball. Flat casebacks on a Tank sit up on the wrist bone and look slabby in profile. Slightly domed examples disappear under a cuff, which is what the design is for.

Details that separate a close copy from a rough one

• Proportions. The ratio between case length and width should stay close to the original. Stretched or squashed cases are the fastest tell in photos.
• Dial printing. Roman numerals are crisp on the Must, with even spacing around the chemin de fer minute track. Fuzzy edges or a track crowding the numerals show up immediately at arm's length.
• Hands. Sword hands need the right length and a proper blue tone. Painted or flat grey hands look wrong against a silvered opaline dial.
• Hidden signature. Cartier tucks its name inside one of the Roman numerals. A copy that omits it, or prints it so heavily it reads from across a room, is not a careful build.
• Crown. The blue cabochon should sit flush and centered. Off-center stones and visible glue haze are common on cheap runs.
• Bracelet and strap fit. On the steel bracelet version, endlinks should meet the case with no gap and no step where metal meets metal.

Movement choices at this size

The genuine XL uses an automatic movement with a power reserve around 40 hours. Copies come three ways at most: basic quartz, an Asian automatic, or a Swiss-parts automatic. Quartz is accurate and cheap to run but feels dead when you pick it up, and the seconds hand ticks once per second. That tick is the biggest giveaway on a dress watch. If the seconds hand sweeps smoothly, the watch is mechanical, or the hand has been deleted on purpose to hide the issue.

If you go mechanical, find out who can service it. Many replica movements have no parts network in the US, so a watchmaker may decline the job or quote more than the watch cost. Better to know that before the watch stops than after.

Straps, wear, and daily reality

The 31mm Tank Must suits a slim calf or alligator strap, and the quick-release system on the original makes swapping simple. Replicas often use fixed spring bars, which pins you to the strap it shipped with. Gold-tone plating wears through at the clasp and caseback first, and it wears faster than most buyers expect. Water resistance is nominal at best on any copy, so keep it out of pools and showers no matter what the listing says.

Caveats worth stating plainly

A replica is not a Cartier. No warranty, no service history, and effectively no resale value. Selling a copy as genuine is illegal in the US, and the trademark belongs to Cartier, not to the factory that made it. If what you actually want is the real thing, the steel Tank Must XL sits at the accessible end of Cartier's range, and a pre-owned example holds value in a way no copy will. Buy a replica if you like the look and accept those terms. Buy the original if you want something that can be repaired and kept.