NB This post contains LOTS of spoilers. Be warned!
The inventiveness of Going Postal is extraordinary, but perhaps the most impressive of Pratchett’s inventions in this book is the Clacks. Going Postal is about all sorts of things, but one of its themes is the battle between the old-fashioned postal service and new-fangled ways of communication. Pratchett can create a powerful Victorian postal service just by writing about it, and in Going Postal there are many accounts of the Ankh-Morpork Post Office in its pomp, when, according to legend, you could post a letter to yourself in the main Post Office on Broadway and even if you ran all the way home, you would still have a job to beat your letter. (“Once….we were postmen!”) But he still needs an opponent to it, something which has taken its business, and he finds it in the Clacks.
The Clacks is basically an old-fashioned semaphore system, such as was briefly used in reality in Napoleonic times. It’s a brilliant solution to the problem Pratchett has, that he can’t use anything which relies on electricity or radio waves, as the Discworld doesn’t have these. He could possibly have a system that relies on magic, but one of the rules of the Discworld is that it’s inadvisable to use magic for everyday purposes (unless you live in Lancre, which is positively stiff with the stuff, and even then most magic isn’t magic but what Granny Weatherwax calls Headology, the witches of Lancre being good at magic but also at herbal medicine and fully au fait with the powers both of suggestion and of the placebo effect.) But the Discworld can do semaphore, and so Pratchett creates the Clacks and turns it into an amazing, continent-wide machine.
The Clacks isn’t the only thing which destroys the Post Office, or even, perhaps, the main thing: that was The Sorting Machine, a machine built by the Discworld’s famous designer Bloody Stupid Johnson to sort the mail. This has at its heart a flywheel with a radius calculated using a pi of 3, because B.S. Johnson thought that actual pi, not being a whole number, was untidy. As Moist von Lipwig exclaims “But you can’t change pi – it’s built in! You’d have to change the universe!”, and as Tolliver Groat replies “Yes, sir. They tell me that’s what happened.” The universe around the heart of the Sorting Machine isn’t the same as the rest of the universe, with unfortunate results.