Long before the birth of social media and dating apps, erotic pocket watches - depicting graphic sexual encounters on the caseback - were given to women by daring suitors. Today, modern interpretations of the centuries-old tradition, including wristwatches, are as highly sought after by collectors as their antique counterparts.

"The interest in erotic pocket watches at auctions is high whenever we have such lots," says Thomas Perazzi, head of Christie's Geneva Watch Department. "We have collectors at Christie's who only collect erotic pieces - both pocket and wrist watches. Every antique pocket watch features paintings on the dials that are unique and thus pieces of art."

A Piguet & Meylan pocket watch made for the Chinese market, for example, was auctioned for about US$411,940 in 2004 at Christie's Geneva sale, fetching well above the estimated price. Other prominent erotic pocket watches created by Henry Moser & Cie, Jaquet Droz and Charles Reuge also showed promising sales at auctions.

The popularity of erotic timepieces dates back to the 18th century and likely stems from the practise of presenting loved ones with memorable keepsakes. At the time, the wives of Royal Navy veterans would have their own portraits engraved onto pocket watches for their globe-trotting husbands.

The earliest known timepieces showcasing imagery of a more graphic nature, however, surfaced around the period of Louis XV of France, circa 1720s.

"Timekeepers with erotic scenes are part of the history of watchmaking," says Patrik Hoffmann, CEO of Ulysse Nardin. "Some of the manufactures decorated the caseback with engraved or hand-painted scenes while others brought the pictures to life on the dial."

Today, high-end watchmakers are even more imaginative with their designs for erotic timepieces, and these watches are certainly not for the faint-hearted.

Ulysse Nardin's Hourstriker Erotica Jarretiere, for example, features an engraved automaton on the dial that springs into action as the striking gong counts the hours.

If that seems like too much excitement for some, there are other modern versions that have opted for a more subtle interpretation, by hiding a "naughty" dial at the caseback. Blancpain's Le Brassus Erotic Minute Repeater Carrousel, for example, features an elegant white grand feu enamel dial accentuated with Roman numeral hour marks and carrousel at six o'clock. The caseback, however, features an embracing couple during intercourse.

These dual dials are an ingenious and popular way of allowing the wearer to display a perfectly presentable watch in public, while enjoying the secret of knowing that there is a saucier version hidden in the caseback. Andersen Genève's ultra-exclusive series, "The Eros", is particularly prized for this feature, and the brand is also renowned for its custom enamelled work in erotic automatons.

Svend Andersen, the veteran who worked at prestigious houses such as Patek Philippe and Gübelin before going independent, was first approached in 1997 by an Italian collector who wanted to have a wristwatch inspired by an antique pocket watch featuring erotic scenes.

Now Andersen has developed a following across the globe. He is working on two commissioned pieces for Chinese customers who came with their own sketches for the design.

While graphic displays have their cheeky appeal, some brands decided to take a more literal approach. Rather than rely on moving figures to send the message, luxury Swiss watchmaker Richard Mille rendered saucy lines of flirtations into an innovative mechanism for the RM69 Erotic Tourbillon.

Three rotating gear trains crafted in Grade 5 titanium are engraved with a number of messages, such as "I long to explore your lips", "Let me kiss you tonight" and "I want to caress you madly".

"Erotic watches have a long history and cultural [relevance]. [Today], people love the watches because these are pieces that make conversations," Mille says.

"I want to do something that's 'tongue-in-cheek' and [involves] a lot of technical aspects. As I always say, I have one foot in the 19th century and another foot in the 21st century."

The excellent craftsmanship as well as technical complications are what underline the everlasting appeal of erotic timepieces to discerning collectors.

"[Erotic scenes] are always painted in high-quality enamel," Perazzi says. "The level of technique in automaton complications is also high, especially considering the fact that they were made in the 19th century."

The modern interpretations raised the technical side of erotic timepieces to a new level. The RM69, for instance, designed in collaboration with Audemars Piguet Renaud & Papi, runs on a hand-wound calibre comprised of an impressive 505 parts. It features a time suspending function which at a push of a button allows the owner to play with the explicit graphics created by the three engraved titanium rollers.

The piece also features a tourbillon and boasts a 69-hour power reserve.

Andersen of Andersen Genève took inspiration from vintage calibres for the erotic automaton and worked magic with his "Eros" series.

The calibres were modified from vintage movements he acquired during the quartz crisis. The calibres were originally designed for alarm clocks.

"The movements were made in the '50s or '60s," Andersen says. "I bought 350 of these movements. My atelier is filled with these parts. I tested a few automatons before and thought they were too thick thus not so elegant. It wasn't until I found the vintage parts that I made the piece work."

Often paired with high-end complications such as minute repeaters and tourbillons, erotic watches today come in limited quantities which are coveted by savvy collectors. The Richard Mille RM69, for example, is limited to 30 pieces and is retailing for HK$5.762million.

Ulysse Nardin's Hourstriker Erotica Jarretiere is available in a limited series of 28 pieces for the 18ct rose gold version and 28 pieces for the 950 platinum version.

The brand is planning to launch a new series of minute repeaters with erotic depictions, and this is due to the immense popularity of the genre.

"The Erotica Hourstriker doesn't only appeal to those who collect erotic pieces," Hoffman explains.

"But also watch lovers who like to be different and appreciate the good things in life."

Andersen's "Eros" series features unique pieces that cater to each individual customer's taste and design. The lead time is around one year and prices range from 59,000 Swiss francs to 79,000 Swiss francs (HK$471,700 to HK$631,590 ). The independent watchmaker makes only about eight unique watches every year.

"Pocket watches without any restorations and in great condition are so rare that we are not able to get them for every auction," Perrazi says. "Today's erotic watches also come in limited editions. In the long term, they could be great investment pieces."

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