If you have followed watch collector conversations and their social media accounts in recent years you might have noticed that quite a bit of the buzz around complications and the sheer number of parts that comprise a watch has faded and attention turned to the appreciation of a ‘hidden complication’, the extra-plat or ultra thin watch.
‘Hidden’ because it doesn’t offer any additional functions, yet comes with several challenges which become apparent only on closer view. Yet, a very thin watch is often considered the epitome of elegance, and hence has inspired watchmakers since long.
When a son wants to prove his worth to his father…
The family name of ‘Piguet’ is an illustrious name in watchmaking in the Vallée de Joux, having brought forth some of the greatest watchmakers, like Grand Complication specialists Victorin Piguet or Louis Elysée Piguet. The latter also shares the honour of having founded the renowned movement maker Frédéric Piguet in Le Brassus in 1858.
Louis Elysée’s son Henri-Louis Piguet followed his father’s professional steps, but wanted to forge his own path by developing an ultra-thin watch movement. As history now records, he succeeded. After almost 15 years of development and improvements the Cal. 21 (back then under its original name ‘Cal. 99’) made its public debut in 1925.
While Henri-Louis considered a movement as thin as 1.50mm, he ultimately settled on 1.69mm to achieve a good balance between absolute thinness and practical considerations like robustness (watches were a necessary tool back then after all) and manufacturing constraints. That original movement – the Cal.99 – had a diameter of 9 9/12” or 20.40mm, 18 jewels, ran at 18,000 vph (2.5 Hz), and a 42hr power reserve.
Thirty years later, in 1955, the escapement was modified to 21,600 vph (3Hz), the bridges redesigned to their current ‘stock’ shape, and a shock protection system added. This resulted in a height increase to 1.73mm, and the movement finally acquired its modern nomenclature: the Cal. 21.
The Cal. 21 held the record for the slimmest wristwatch movement in the world for more than two decades, has been used by most watch manufacturers, and remains in production today under the Swatch Group (which acquired Frédéric Piguet in 1992).
The construction of such a slim movement is a remarkable feat, just imagine a slight horizontal tolerance error on a wheel bearing. While such might be inconsequential on a ‘normal’ movement like the ETA 2801, the same error on a Cal. 21 would result in twice the degree of angle error which could well be enough for the teeth to no longer mesh, the wheels to jam into bridges, and so on. That Frédéric Piguet successfully industrialised this movement a century ago is commendable given the then available machinery and tools (i.e.: no CAD, CNC or the like), and largely causative for the accolades this movement still earns today.
Transforming a delicate movement
The opportunity to work with such a movement, and to add his take on it, was a challenge too good to pass up for MT. Such movements open design possibilities not available elsewhere – not only for the watch but also for the movement itself…
The only way to acquire a Cal. 21 (for a brand outside the Swatch Group) these days is patience, knocking on a lot of doors and running into a bit of luck in finding a vintage batch in good condition. Once the stars aligned and the movements were ours, MT set about reworking the Cal. 21 completely “making it look clean and modern, and in line with the open and layered aesthetic of our other movements”. His re-interpretation respects the origins of the movement while evolving it for MING and modern watchmaking.
MT’s design brief for the now Cal. FP21.M1 included new bridges, replacing the classic Vallée-de-Joux-style with more dynamic and bolder barrel and escapement bridges. The train bridge is now machined in titanium, open worked and blue coloured for maximum visual impact. The finishing contrasts a fine sandblasting of the top surfaces with a diamond cut anglage for added depth.
MT wanted the ratchet wheel in our signature spoke design and given the very precise tolerances required, we had to manufacture brand new ratchet wheels. In the end, only the base plate and most of the round components remain stock.
The upgrades were not exactly straightforward. For starters, no technical drawings were available, much less 3D files. Time-consuming disassembly and manual measurements of salient parameters were needed to obtain necessary data points. The desired blue hue was only obtainable on titanium, (and we seem to like making our lives harder) so we chose this material for the new skeletonised gear train bridge. Originally, MT wanted our signature bicolour bridge finish with bright rhodinised anglage, however, the very thin bridges made this impractical. The rubies had to be set in chatons to allow for secure fitting of stones and some degree of manual adjustment for each individual movement – all too often vintage parts and modern precision manufacturing doesn’t work together well. As a bonus, the chatons also add a bright sparkle right in the deep blue…
The colourway of the souscription movement is certainly that of a ‘celestial sparkle’, but for the series watch, MT opted for something more classical and warm – 5N coated bridges that echo the finishing of some of our Agengraphe, La Joux-Perret or Schwarz Etienne movements. There is a slight twist however, as only the top bridges are clad in 5N coating and the baseplate remains the same as its sibling enhancing visual depth and layering of the movement in person.
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To make all these modifications happen, the expertise of TitAl SA in Yverdon-les-Bains (best known for their Titalyt® treatment of F.P. Journe’s Élégante cases) and Mathis Horlogerie in Les Brenets, specialists in vintage movement restoration, was called upon.
So here we are with a remarkably thin movement with a stunning vertical structure that doesn’t’ give away its age.
Happy 100th birthday, Cal. 21!
– Dr. B.
Author’s note: I would like to thank Jacques Frédéric Piguet for valuable input. More details (in French) on the Piguet family of famous watchmakers and sportspeople in Le Brassus can be found at https://piguet-famille.ch.
Published: 6th March 2025