Casio F-91W

The F-91W is a small rectangular digital watch in black resin, about 38 millimetres by 35 and less than a centimetre thick, weighing 21 grams. A grey liquid-crystal display sits in a printed bezel; three steel buttons on the sides work the light, the modes and the alarm. It shows the time and the day, counts to a hundredth of a second, beeps on the hour, and runs for about seven years on a single coin cell.

Casio introduced it in June 1989 to replace the F-87W. It was the first watch designed for the company by Ryūsuke Moriai, who went on to work on the G-Shock, and it was meant to be small, legible and durable for very little money. It has stayed that way. In 2011 the BBC found it selling for as little as seven pounds; the company would not give figures but called it a huge seller everywhere.

That ubiquity gave the watch a strange second history. In 2011 leaked files from Guantánamo Bay showed American interrogators treating ownership of an F-91W as a sign that a detainee had learned to build time bombs; more than fifty detainee reports referred to it, though millions of people with no such training wore it. A smaller oddity concerns its date. For years almost every source gave its launch as 1991, a figure that began as an unsourced edit to Wikipedia and spread through the press until a 2020 investigation, and Casio America's own records, put it back to 1989.

The watch sold today is essentially the one sold then. The amber microlight of early examples has become a green LED, but the case, the module number stamped on its back, the buttons and the functions are unchanged. It is one of the few objects in this catalogue that costs less than a meal, and one of the most widely copied.