on. Known for its simple and elegant design, the Calatrava wristwatch has been a flagship model of Patek Philippe since its introduction. Notably, the Calatrava Cross has been company's logo since 1887. World Time wristwatch Patek Philippe World Time (Heures Universelles) collection was introduced in 1939, with Ref.1415 being the first model. The "World Time" complication is able to show the time for all 24 time zones on the same watch and was invented by Swiss watchmaker Louis Cottier in 1931. Cottier's invention attracted several watchmaking companies, while Patek Philippe was the first company to introduce a series of World Time wristwatches after forging a partnership with Mr. Cottier. The successors to Ref. 1415 include Ref. 2523, Ref. 5230, Ref. 5531 (with minute repeater), and so on. As of 2018, Patek Philippe World Time collection (Ref. 1415, Ref. 2523) holds 6 spots among the 58 world's most expensive watches sold at auction (over 2 million US dollars), with the world record being 4.027 million US dollars (6,603,500 CHF) made at Antiquorum's Geneva auction on April 13, 2002. Nautilus wristwatch In 1976, Patek Philippe introduced the Nautilus collection after deciding it was time to produce an exclusive sport watch with finishes of the highest quality. The first model was Ref. 3700 and was made of steel. It was designed by Swiss watchmaker Gérald Genta, who previously designed the Royal Oak collection for Audemars Piguet, and was released by Patek Philippe during the quartz crisis in the hope that it would help re-attract people's attention to high-end Swiss mechanical watches. The Nautilus collection played a key role in Patek Philippe's overall marketing strategy as it had to refresh the brand image while perpetuating tradition. The target was represented by dynamic business managers of the new generations. The Nautilus wristwatch has become one of the most popular collections from Patek Philippe, and the Ref. 5711 & 5712 models, which the company introduced in 2006 to celebrate the 30th anniversary of the collection, are among the most popular models. Perpetual calendar chronograph Patek Philippe perpetual calendar chronograph was introduced in 1941, with Ref. 1518 being the first model. On November 12, 2016, a Ref. 1518 in steel became the most expensive wristwatch ever sold at auction (until October 2017), fetching a record-breaking US$11.14 million (11,002,000 CHF) in Geneva (through Phillips' auction house). As the successor, Ref. 2499 is widely considered to be one of