The WSSA0018 is the large steel Santos de Cartier, introduced in 2018 when Cartier renewed the Santos line. The shape goes back to the watch Louis Cartier designed in 1904 for the aviation pioneer Alberto Santos-Dumont: a square case with a screwed bezel that flows into the lugs.
The case measures 39.8 by 47.5 mm, the silvered dial has Roman numerals and blued steel sword hands, and the crown carries a cabochon.
Inside is the automatic calibre 1847 MC, with a power reserve of about 42 hours. The bracelet has the SmartLink system, which lets you remove or add links without tools, and QuickSwitch lets you exchange the bracelet for a leather strap without tools.
On a pre-owned example we look above all at the polished bezel and its screws, where scratches and polishing marks show quickly, at how QuickSwitch and SmartLink work, and at the completeness of the set: box, guarantee card, any second strap and spare links.
For a smaller wrist there is the medium WSSA0029. What a complete set includes is explained in our guide to box and papers.