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The Clacks is the informal nickname for the semaphore system that is the fastest means of communication on the Discworld. It was first seen in The Fifth Elephant. The Grand Trunk Semaphore Company runs the system.

The machinery consists of a series of towers, each of which contains a grid of eight shutters, more for particularly busy towers. By opening and closing these shutters, very complex messages can be coded and sent long distances - it is even possible to send colour pictures. The system was invented by Robert Dearheart. The shutter machinery can be operated by humans, but Moist von Lipwig in Making Money mentions treadmills powered by donkeys, and suggests that Golems might be easier to get up into the towers.

The original machinery has been improved upon by the use of clockwork and punch cards known as jacquards. Clacks operators typically tend to be single-minded to the point of obsession. Sometimes Gargoyles are hired as clacks operators, especially in the city, because of their ability to sit patiently on rooftops.

It is the Discworld's approximate equivalent of the Internet. Several companies and organisations in Ankh-Morpork have their own towers and "c-mail" addresses, including the Ankh-Morpork City Watch, We-R-Igors and Unseen University. The latter's semaphore tower is linked up to Hex, effectively creating the Discworld's first modem. Companies typically use their own codes in addition to the Grand Trunk's own. There was also a brief fad for portable semaphore sets, but this has not appeared since its mention in the Thieves' Guild Yearbook. The Watch has experimented with semaphore helmets, with limited success.

As with this world's Internet, it has brought new types of crime with it. Hacking (i.e. breaking into a semaphore tower) arose fairly quickly, but this form of computer crime was dealt with by the simple expedient of arming the tower operators. The Smoking Gnu discovered that certain codes could disrupt and damage the delicate machinery, something they often did in protest at the policies of the Grand Trunk Semaphore Company.

The Grand Trunk

This was a system of towers founded by Robert Dearheart and runs from Ankh-Morpork to Genua, via Überwald and Borogravia. The towers in the wilder parts of the continent were typically fortified. Ankh-Morpork would go to war against any country who threatens the traffic on the Grand Trunk, which it did in Monstrous Regiment. (Or, more accurately, go to the war and explain to the combatants that it would be very bad for their commerce and industry to threaten the Clacks. Ankh-Morpork has not had an army in living memory.)

Unfortunately, the pioneers who invented it were not as good at running a business as they were at engineering, and were bought out using embezzled money. The Grand Trunk came under the control of the corrupt Reacher Gilt. The new company eliminated maintenance time, resulting in the Clacks often being down for major repairs.

Lord Vetinari has recently (in Going Postal) intervened following exposure of the Company's crimes.

Historical Counterpart

The real-world counterpart of the Clacks was known as the optical telegraph or Semaphore Line. Invented in the late 18th century and operated into the early 19th century before being made obsolete by electrical telegraphy, semaphore lines were used by the governments of France, Britain, and other European countries to convey vital information more rapidly than horseback riders could. Semaphore lines could only send about two words a minute, and were thus much less efficient than those of Discworld.