Launched in 1991 when the smartphone was still almost a decade away - the first GSM digital network deployed the same year, so this was right at the start of the digital era of mobile communications - Casio continue to sell the watch to this day. While sales figures have never been released (Casio simply say it remains a 'huge seller') you could easily argue that this is the biggest selling watch in the world. It's certainly recognisable, functional, and opened up accurate digital timekeeping to almost every corner of the planet.
Based around a comparatively large LCD screen, the F-91W packs a lot of information on its home screen. Not only do you have the time as a twelve or twenty-four hour clock with the seconds also on display, you also have the day and date, as well as indicators for your alarm and hourly chiming. With an all-plastic construction and shipping with a plastic resin band (but using the standard connections if you want to switch it out) the F-91W weighs just 22 grams. There is a touch of tension around the wrist because of the band, but in terms of weight the watch is not noticeable.
The standard version is marked as 'Water Resist' by Casio, It will happily take on board any rain or water splashes during everyday use, but this is not a watch to take in the shower or go swimming with.