Casio F-91W F-91W-1

The storyWhy this reference matters

June 1989. Casio needs a successor to the F-87W and hands the job to a young designer, Ryusuke Moriai; it's his first project for the company. Out of it comes a rectangular resin case eight and a half millimetres thick, a steel back held by four screws, and module 593 inside. Nearly forty years later the brand still makes exactly that, at a rate of some three million units a year, and the running total has passed a hundred million. It is, by a distance, the best-selling watch in the world.

Nobody at Casio went looking for the F-91W's second life. In 2011 the files on the Guantánamo detainees leaked and one phrase kept coming up: wearing a Casio F-91W, or its metal cousin the A159W, was listed as an indicator in the records of more than fifty prisoners. The argument was that the module had been used as a timer in explosive devices, documented in the Ramzi Yousef case in 1994 and Ahmed Ressam's in 1999. A twenty-one-gram watch turned into evidence.

And all the while the same watch was on the other wrist. CIA officer J. R. Seeger told Fratello he wore an F-91W in Afghanistan in 2001 and later moved to a G-Shock: small, discreet, and if you lose it or destroy it, nothing happens. That double life explains rather well why Casio itself sells it today under the vintage digital watch label, without touching a comma of the design.

SpecificationsThe numbers, in one table

IdentificationHow to tell it apart

The F-91W-1 is the original black one, the one with the blue detail. Every later run — the monochrome WC of 2010, the two-tone metallic WM of 2018, the translucent WS of 2020 — changes the colour and nothing else: inside they all carry the same module 593. Turn it over and check the steel back, held by four screws with the module number stamped on it.

The detail almost nobody looks at: the 593's calendar has February fixed at 28 days. Casio's own manual says so in as many words. That means every leap year, on 1 March, your F-91W runs a day ahead and has to be corrected by hand. It isn't a fault or a broken example: it's how the thing has been built since 1989, and it's a quick test for weeding out copies that promise a perpetual calendar.

Official figures to check any listing against: accuracy of ±30 seconds a month, stopwatch to 59'59.99 reading to the hundredth, daily alarm, hourly signal, 3 bar of water resistance and a CR2016 cell with a claimed seven years of life. The case measures 38.2 × 35.2 × 8.5 mm, weighs around 21 grams, and the lug width is 18 mm — which is why half an industry of NATO adaptors exists for this watch.

CollectingWhat collectors check

There's no investment case here: three million are made every year. What there is, is judgement in picking a used one. The strap is the first thing to die (it hardens and snaps next to the case), so discount that replacement from the start. Then look at the display head-on and from the side: weak segments or a grey veil mean the connector between module and screen is on its way out, and on this watch it isn't worth touching.

The lens is acrylic, so it scratches if you look at it and polishes out with a bit of paste over an evening on the sofa. The steel back and its four screws make a battery change trivial, but mind the part almost nobody replaces: those 3 bar depend on one tiny gasket. If you're going to get it wet, change it. Fratello took one swimming and it came out unharmed, but that isn't Casio's guarantee.

A note on wearing rather than buying: the F-91W's light is bad, and that's that. It's a lukewarm green LED that won't get you out of a cinema. If that matters to you, the same house makes the metal A1000 family, with module 3479 and equivalent specification. If it doesn't, no watch disappears on the wrist faster.

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