The clacks in Terry Pratchett's Discworld novels is a network of semaphore towers stretching along the Sto Plains, into the Ramtops and across the Unnamed Continent to Genua. It was introduced in The Fifth Elephant and has become the Discworld's first telecommunications network. While the system structure is that of a telegraph, elements of it are often described as similar to the Internet; for example, it threatens to make the Post Office obsolete in Going Postal (which also features a group of clacks hackers) and is sometimes described as 'c-mail' (a clear reference to e-mail). 'C-commerce' is also carried out on it.
A possible influence for the clacks system is the similar semaphore network in the Keith Roberts novel Pavane or the hoodwinker towers in The Blue World by Jack Vance. Both are based on the real-world optical telegraphs used in the early 19th century before electrical telegraphy made them obsolete. A similar telegraph system is described in the L. Sprague de Camp novel Lest Darkness Fall. The name itself may have been inspired by 'clackers', the term for operators of mechanical computers in William Gibson and Bruce Sterling's steampunk novel The Difference Engine; it is also a play on the word 'fax'.
The ground floor is a storeroom, the second contains an office, a kitchen and, in out-of-the-way towers, a bunkroom. The top floor contains the controls, two chairs face identical control boards on either side, each connected to the panels on the opposite side. There is a keyboard, levers, and pedals. Sometimes entering a code will alter the configuration of the system, probably beneficially (but see Smoking GNU, below.)
Based on this, he founded the Grand Trunk Company which began creating a network of towers that would stretch across the continent. "High traffic" towers have more than six panels. The largest is the one on the huge hill in Ankh-Morpork called 'The Tump' (a cross between the Trump Tower and the Post Office Tower), which is the main junction between the city's clacks network (various city institutions, including the Guilds, and the Watch had installed small clacks towers on their buildings) and the chain of towers that leads past Sto Lat, into Überwald, and from there to Genua. In the less civilized areas in the heart of the continent, they ran into problems, and most clacks towers in the Überwald area had fortified stone bases and, often, armed guards. In Monstrous Regiment a particularly acute problem in Borogravia is described, where the towers were seen as an 'Abomination unto Nuggan' (the local god), on the grounds that if messages were being sent through the air, prayers would get tangled up in them.
Because much of the material being sent was confidential, the senders would put it in their own code before being given to the clacks operators. The operators were, therefore, often surprised if they received a message they understood, outside the Overhead (the messages from and about the network itself).
The Grand Trunk employed a lot of gargoyles as they were exceptionally good at sitting and watching without getting bored.
The success of the clacks network resulted in a fad for semaphore of all types, and fashionable Morporkians began carrying signal flags with them, to send messages to friends on the other side of the room. This appears to have died out, although the City Watch has its own semaphore network, with a relay station on the roof of the Old Lemonade Factory (the Watch training school).
Dearheart, and his employees, continued to improve the network, and as the network grew larger, activating the shutters directly became too complicated, so methods of automating the process were introduced. Punch cards, nicknamed jacquards, were designed that would send certain messages automatically, and clockwork machinery was added to regulate the mechanisms. Outgoing messages were stored on rolls of punched paper called "drum rolls" (presumably after the differential drum, which seems to be the centre of the clockwork). They even worked out a way of coding pictures by using numbers to represent colours; this was later simplified, as explained in Monstrous Regiment.
The head of the new consortium was a man named Reacher Gilt. A ruthless businessman with a piratical appearance, including an eyepatch and a parrot (actually a cockatoo that, instead of repeatedly saying "pieces of eight", repeatedly said "twelve and a half percent"), he was a shameless con-artist and fraudster whose business style was described as "Find the lady with entire banks". He maintained his monopoly by killing anyone attempting to set up another network, including Dearheart's son, John, and employing the banshee Mr. Gryle to do so.
Alex Carlton and Al Winton are clacks engineers. Despite Alex's nickname, it is Al's opinion that he's mad as well, because nobody normal would organize screws by size (Jeremy Clockson had the same kind of "sanity", and many Discworld villains have similar afflictions). Adrian Emery is an alchemist, and has a number of ideas for improving the clacks with chemicals that change colour (and hardly explode at all).
The clacks system has also been cracked by Hex, after Ponder Stibbons connected it to the Unseen University's tower (thereby making Hex the equivalent of a computer with an internet connection). Whether or not this is actually legal is a question the faculty is carefully not asking.
Some c-mails include: