The Patek Philippe World Time model dates back to 1937. It used the World Time system created by independent watchmaker Louis Cottier in Geneva, Switzerland in 1935. Uniquely designed for the world traveler, this line of watches features a rotating ring that includes the names of 24 popular cities. The watch also has a second rotating ring that used to display the time in 24 different time zones via 24-hour markers. Patek created many World Time models dating back to 1937, starting with Ref. 515. These models were followed by some such as 542, 96HU, 1415HU, 1415/1, 1416HU, 2325HU and 2325/1 (discontinued in 1965). In 2000, the firm introduced 37-mm reference model 5110, which was discontinued around 2006 when they introduced Ref. 5130 at the Basel fair with a bigger 39.5mm diameter. The World Time collection is considered by some to be one of the finest in the Patek Philippe lineup, mainly due to its excellent craftsmanship and complications.