The Casio F-91W is a
digital watch
A watch is a portable timepiece intended to be carried or worn by a person. It is designed to keep a consistent movement despite the motions caused by the person's activities. A wristwatch is designed to be worn around the wrist, attached by ...
manufactured by Japanese electronics company
Casio. Introduced in 1989
as a successor of the F-87W, it is popular for its low price and long battery life.
As of 2011, annual production of the watch is 3 million units.
Its ubiquitous use in the construction of timers for
terrorist bombs prompted US officials to view the wearing of these watches as a "sign of
al-Qaeda
Al-Qaeda (; , ) is an Islamic extremism, Islamic extremist organization composed of Salafist jihadists. Its members are mostly composed of Arab, Arabs, but also include other peoples. Al-Qaeda has mounted attacks on civilian and military ta ...
".
Specifications
Design
Designed by Ryusuke Moriai as his first design for Casio,
the case of the F-91W measures . The case is primarily made of
resin, with a
stainless steel
Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's corros ...
caseback and buttons with the manufacturer's module number, 593, stamped on the caseback. The
resin strap is at the fitting and 22 mm across the widest part of the lugs. The total weight is .
Features
The F-91W is a
chronograph, featuring a second
stopwatch with a count up to 59:59.99 (nearly one hour). The stopwatch also has the feature to mark net and split time (lap). Other features include an hourly time beep and a single daily alarm lasting 20 seconds and an annual calendar, with adjustment for
leap years not supported as the watch does not record the year. February is always counted as 28 days.
The watch uses a faint, green
LED
A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
backlight located to the left of the display for illumination (in earlier versions it was an amber microlight). According to manufacturer estimates, the watch is reported to be accurate to ±30 seconds per month.
The
quartz movement
Movement may refer to:
Common uses
* Movement (clockwork), the internal mechanism of a timepiece
* Motion, commonly referred to as movement
Arts, entertainment, and media
Literature
* "Movement" (short story), a short story by Nancy Fu ...
, designated ''Module 593'', is powered by a single
CR2016 3-volt
lithium button cell.
Water resistance
The watch front is marked "
Water Resist",
but Casio reports different values for different variants of the watch. The black version (F91W-1)
is "30 meter / 3 bar" (i.e. 100 feet / 44 psi), the ISO standard meaning of which is: "Suitable for everyday use. Splash/rain resistant. NOT suitable for showering, bathing, swimming, snorkeling, water related work and fishing".
Operation

The watch is controlled by three side-mounted push-buttons.
*The upper left button, labeled "Light", turns on the light, cancels the alarm, resets the stopwatch or marks the split (lap) time, and is used for selecting settings.
*The lower left button, labeled "Mode", cycles the modes of the watch: time display, alarm, stopwatch, and time/date adjustment.
*The right button, labeled "Alarm On-Off/24hr", is the function button: when used, it starts and stops the stopwatch, changes the settings currently being adjusted, or switches between the 12- and
24-hour
The modern 24-hour clock, popularly referred to in the United States as military time, is the convention of timekeeping in which the day runs from midnight to midnight and is divided into 24 hours. This is indicated by the hours (and minutes) pas ...
modes, depending on what mode the watch is currently in. Pressing all three buttons at the same time will fill all the cells on the LCD until any button is pressed again.
The time or date is adjusted by pressing the lower left button three times to bring the watch to time adjustment mode. The top left button is used to cycle through seconds, hours, minutes, month, date, day, and normal mode. The right button is used to adjust the flashing value displayed. Unlike any other value, the seconds can only be zeroed. Should this happen before 30 seconds, the watch will zero in at the beginning of the current minute. After 30 seconds it will start the next minute as displayed. When the adjustments are finished, the bottom left button can be pressed once to return the watch to normal mode.
The watch display shows the day of the week, day of the month, hour, minute, seconds, and the signs PM in the afternoon – or 24H (24-hour clock) – at all times, the alarm signal status (bar of vertical lines), and the hourly signal status (double beep on the hour, shown as a bell) are present when activated in the alarm mode.
In stopwatch mode, minutes, seconds, and hundredths of a second are shown.
Usage in terrorism

According to secret documents issued to interrogators at
Guantanamo Bay, obtained and released by ''
The Guardian'', "the Casio F-91W digital watch was declared to be 'the sign of
al-Qaeda
Al-Qaeda (; , ) is an Islamic extremism, Islamic extremist organization composed of Salafist jihadists. Its members are mostly composed of Arab, Arabs, but also include other peoples. Al-Qaeda has mounted attacks on civilian and military ta ...
' and a contributing factor to continued detention of prisoners by the analysts stationed at Guantanamo Bay. Briefing documents used to train staff in assessing the threat level of new detainees advise that possession of the F-91W and the A159W – available online for as little as £4 – suggests the wearer has been trained in bomb making by al-Qaeda in Afghanistan."
United States
military intelligence officials have identified the F-91W as a watch that terrorists use in constructing
time bombs.
[
]
This association was highlighted in the
Denbeaux study Seton Hall report, also known as the Denbeaux study, is any of several studies, published by the Center for Policy and Research at Seton Hall University Law School in the United States beginning in 2006, about the detainees and United States governm ...
, and may have been used in some cases at Guantanamo Bay.
An article published in ''
The Washington Post'' in 1996 reported that
Abdul Hakim Murad,
Wali Khan Amin Shah, and
Ramzi Ahmed Yousef
Ramzi Ahmed Yousef ( ur, , translit=''Ramzī Ahmad Yūsuf''; born 20 May 1967 or 27 April 1968) is a Pakistani convicted terrorist who was one of the main perpetrators of the 1993 World Trade Center bombing and the bombing of Philippine Airline ...
had developed techniques to use commonly available Casio digital watches to detonate time bombs.
[
] Casio watches were mentioned almost 150 times in prisoner assessments from Guantanamo.
On July 12, 2006, the magazine ''
Mother Jones'' provided excerpts from the transcripts of a selection of the Guantanamo detainees.
The article informed readers:
More than a dozen detainees were cited for owning cheap digital watches, particularly "the infamous Casio watch of the type used by Al Qaeda members for bomb detonators."
The article quoted
Abdullah Kamel Abdullah Kamel Al Kandari:
When they told me that Casios were used by Al Qaeda and the watch was for explosives, I was shocked... If I had known that, I would have thrown it away. I'm not stupid. We have four chaplains t Guantanamo all of them wear this watch.
Variants
Counterfeits
Counterfeits of this watch are very common, despite its low price tag. These counterfeits generally have a lower plastic build quality, narrower LCD viewing angles, louder and higher-pitched beeps, and significantly less accurate timekeeping than genuine models.
The newer modules with the green LED light can be tested by holding the right button for over 3 seconds in the main timekeeping mode; this will lead the display to show "CASIo", as a test for authenticity. With the advancement in technology, however, some counterfeit models have also been developed to show this sign, although these are fairly uncommon. This leaves the only method of distinguishing them as assessing the overall build quality, timekeeping accuracy, display viewing angle and the printing on the screen.
References
External links
*
F91W-1 product page Casio website
{{DEFAULTSORT:Casio F91w
F91W
Products introduced in 1989