"World Time System". This system was invented around 1935 by Louis Cottier (1894 - 1966), a famous independent watchmaker in Geneva. This World Time system was used by Patek Philippe and also by other major names in the Swiss watch industry, such as Agassiz, Rolex and Vacheron Constantin. Patek Philippe created various series of ?World Timers? from 1937 to the mid-1960s: Ref. 515, a rectangular ?galbé? pink gold model made in 1937, with the names of 28 cities in the world on the dial, the time differences between them based on Greenwich Mean Time. It is the earliest known and only three examples are known today. One was sold by Antiquorum, in Geneva, on April 10, 1994, lot 502, for SFr. 550,000 (US$ 388,560). Ref. 542 HU (Ø 28 mm.), started around 1937, bears the names of approximately 30 cities in the world on the revolving bezel. Only 4 examples are known to date, three of which were sold by Antiquorum: - Geneva, October 17, 1993, lot 431, at SFr. 121?000 (US$ 81,700). - Geneva, April 10, 1994, lot 500, at SFr. 231'000 (US$164,200) with unusual bezel and hands. - Geneva, October 15-16, 1994, lot 608, at SFr. 110'000 (US$ 87,300). Ref. 96 HU Calatrava, with the names of 28 cities in the world on the dial, appears to be a unique example made in 1939. Ref. 130, of which only one example is known and was made on special order by Patek Philippe in 1939, is an 18K yellow gold mnemonic ?World Time? chronograph wristwatch, with square push buttons, register and tachometer. This watch does not have a ?World Time? movement but features a ?World Time? dial with the names of 24 cities of the world. The time difference between the various cities is based on Berlin, which is placed at noon. This watch was sold by Antiquorum, in Geneva, on April 18-19, 1998, lot 40, for SFr. 355?000 (US$ 235,000), establishing a world record for this type of watch. Ref. 1415 HU began to be produced around 1939-1940. It has drop-shaped lugs, a diameter of 31 mm., and on the revolving bezel the names of approximately 30 to 41 cities in the world. This model was produced: - in yellow gold with silvered dial - in yellow gold with cloisonné enamel dial representing various continents - in pink gold, with silver, champagne or pink dial - in pink gold, with cloisonné enamel dial representing various continents - Antiquorum sold such a watch in Geneva, on November 13-14, 1999, lot 520, which set a World Record at SFr. 1?345?500 (US$ 868,065) for this type of model. - in platinum with silvered dial; the present watch is the only one of these known. Ref. 1416 HU is similar to Ref. 1415HU and produced at the same time, with the only difference that it has straight ?claw? lugs. Three examples are known today. Ref. 1415-1 was created in 1940, especially for Dr. P. Schmidt. This is a unique ?World Time? chronograph wristwatch, in yellow gold, with square push buttons and the names of 33 cities of the world. It was auctioned by Antiquorum, in Geneva, on October 15-16, 1994, lot 516, attaining what at the time was a World Record, at SFr. 990'000 (US$ 785,714). This watch is illustrated on the front cover and p. 270 of the new edition (1998) of Patek Philippe Wristwatches, by Martin Huber & Alan Banbery. Ref. 605 HU was produced circa 1940-1960s and is a series of dress watches, with on the revolving bezel the names of approximately 30 to 41 cities in the world. Some examples have a matte silvered dial or black dial and others have a cloisonné enamel dial. Ref. 2523 HU and Ref. 2523-1, with two crowns, were produced circa 1953-1965: they bear the names of approximately 40 cities of the world on the revolving bezel. They were made: - in pink gold, with silvered guilloché dial - in pink gold, with translucent enamel dial - in pink gold, with cloisonné enamel dial representing various continents - in yellow gold, with silvered guilloché dial - in yellow gold, with translucent enamel dial - in yellow gold, with cloisonné enamel dial representing various continents - one example is known to date in white gold, with cloisonné enamel dial representing various continents.