The Discworld is a fantasy land in the Tolkien and Brothers Grimm mould, complete with witches, wizards, dragons, trolls, and dwarfs; however, over time it has largely evolved into its own distinct culture, as its denizens find more sophisticated ways to outgrow their narrative conventions. The Disc is heavily influenced by magic and, while having similarities to (and in some cases, based on) planet Earth, it (generally) conforms to its own laws of physics.
Pratchett first explored the idea of a disc-shaped world in the novel Strata (1981).
Great A'Tuin's gender is unknown, but is the subject of much speculation by some of the Disc's finest scientific minds. The sex of the World Turtle is pivotal in proving or disproving a number of conflicting theories about the destination of Great A'Tuin's journey through the cosmos. If, as the popular "big bang theory" states, Great A'Tuin is moving to his (or her) mating grounds, then at the point of mating might the civilisations of the Disc be crushed or simply slide off? Attempts by telepaths to learn more about Great A'Tuin's intents have not met with much success, mainly because they did not realise that its brain functions are on such a slow timescale. All they've been able to discern is that the Great A'Tuin is looking forward to something. (Telepaths have also attempted to read the minds of the Elephants, who apparently feel 'incredibly bored', and have terrible back pains.)
The other theory, described as being popular among academics is the "steady gait" theory, is that he/she came from nowhere and is going to keep swimming through space to nowhere for ever.
Faust Eric shows Great A'Tuin being made instantly from nothing, seemingly in support of the theory that it came from nowhere and would continue at a constant pace into nowhere; however, the events in The Light Fantastic, in which the Great A'Tuin attended the hatching of eight baby turtles, each with four baby elephants and a tiny discworld of their own, would seem to support the Big Bang hypothesis. A combination of the two theories might be possible, with A'Tuin being among the first generation made by the Creator and subsequent generations being created through breeding.
The little turtles have since gone off on their own journeys. Whether this was the event the Great A'Tuin was looking forward to or merely one step towards its ultimate goal is not mentioned.
Great A'Tuin has been mentioned to frequently roll on its belly to avoid asteroid and comet collisions, or even to snatch these projectiles out of the sky which might otherwise destroy the Disc. These stunts do not affect the Disc's population, other than to induce severe seasickness on anyone who happens to be looking at the night sky at that time. A'Tuin has been known to do more complex rolls and corkscrews, but these are rarer. This is similar to real-world sea turtles' habit of rolling over with their flippers in the air to protect them from sharks.
Due to the Great A'Tuin's travelling through the universe, the night sky of the Discworld, unlike that of our world, changes markedly over the course of decades, as the turtle departs older constellations and enters new ones. This means that astrologers must constantly update and alter their horoscopes to incorporate all-new zodiacs.
A tiny sun and moon orbit the Great A'Tuin, both about 1 mile in diameter when described at the start of the series, but the description of their diameter is increased to at least 80 miles later in the chronicles. The moon is slightly closer to the Disc than the sun, and is covered, on one half, with silvery glowing plants, which feed the lunar dragons. The other half is burnt black by the sun. The moon rotates, and completes a full revolution in about a month; the full moon occurs when the luminescent side is completely visible from the Disc, the new moon when the dark side is shown. The sun's orbit is so complex that one of the elephants has to cock its leg to allow the sun to continue on its orbit.
According to the wizards of Unseen University, chelys galactica are composed largely of the element chelonium, the properties of which are apparently known to them (they do tests to look for it in Roundworld in The Science of Discworld), but not to readers.
The Disc itself is stated as roughly 10,000 miles wide, giving it a surface area two-fifths that of the Earth. Its principal geographic feature, other than its flatness, is the Cori Celesti, a great, 10-mile-high spire of rock that lies at its exact centre and is the point of origin for its standing magical field. The Cori Celesti is also the location of Dunmanifestin, the home of the Disc's many gods, a nod towards Mount Olympus. The area including the Cori Celesti is known as the The Hub, a land of high, icebound mountains that serves as an analogue both to the Himalayas, to Roundworld's polar regions (since, although the Disc has no poles as such, it is as far as possible from the Disc's edge and thus the sun), and to Scandinavia – the Hublanders share many features with vikings. Polar bears are known as "Hubland bears", while the Disc's equivalent of the aurora borealis (described as being produced by the Disc's magical field, rather than by magnetism) are known as the "aurora coriolis." Directions within the Discworld are not given as North, South, East and West, but rather as directions relating to the disc itself: Hubward (towards the centre), Rimward (away from the centre) and to a lesser extent, turnwise and widdershins (respectively, with and against the direction of the Disc's spin).
The areas closer to the Rim are warmer and tropical, since the Disc's sun passes closer to them in its orbit. At the Rim, a great, encircling waterfall (the Rimfall) sends the Disc's oceans cascading into space. Pratchett is evasive about how the water eventually returns to refill the oceans, only saying, "Arrangements are made." The mist from the plunging waters creates the Rimbow, an eight-colour (the eighth is octarine) double rainbow consisting both of light and of magic.
There are four main continents on the Disc, along with a number of geographical and political regions and islands some of which have not been described in detail in the novels and the main information comes from The Discworld Mapp and footnotes. The majority of the Disc's landmass is composed of a single supercontinent comprising a large main region and a smaller Counterweight Continent connected by a narrow isthmus. The main continent comprises the unnamed Continent upon which most of the novels are set, and Klatch, akin to Africa, India and the Near and Far East. The island continent of Fourecks (A.K.A. Terror Incognitia) is the smallest of the four, and resembles Australia. On these continents a large number of countries, kingdoms, cities and towns can be found; the most widely mentioned in the books being Ankh-Morpork, Lancre, the Klatchian Empire and Überwald. In the Discworld Companion, Pratchett writes "there have been other continents, which have sunk, blown up, or simply disappeared. This sort of thing happens all the time, even on the best-regulated planets."
Magic is the principal force on the Discworld, and operates in a similar vein to elemental forces such as gravity and electromagnetism on our own world. The Disc's "standing magical field" is basically the local breakdown of reality that allows a flat planet on the back of a turtle to even exist. The force called "magic" is really just a function of the relative absence of reality in the local area, much in the same way that we describe absence of heat as "coldness." Magic warps reality in much the same way as gravity warps space-time.
On the Discworld, where magic has more in common with particle physics than Houdini, high-level background magic (most likely a reference to background radiation) occurs when a very powerful spell hits, creating a myriad of sub-astral particles that severely distort local reality. Building a house in (or even walking into) a region where this has happened is extremely dangerous as it is mentioned that an individual may not remain the same species, shape or level of sanity, if they are entered. Medium levels cause odd effects, such as coins landing on their edges and turning into caterpillars. Areas with larger than normal quantities of background magic tend to display unusual qualities, even for the Disc. Very high quantities of magic can knock a hole in reality, leading to an invasion by Lovecraftian monstrosities from the Dungeon Dimensions, or, almost as bad, the world of the Elves. On the Disc, magic is broken into elementary particulate fragments in much the same way that energy and other forces are in quantum physics. The basic unit of magic is the thaum, but the thaum is in turn made up of particles known as "resons" (literally, "thingies") or reality fragments. These are in turn composed of five "flavours": up, down, sideways, sex appeal and peppermint (as spoof of quarks).
In the opening books, the number eight is generally significant and has magical properties on the Disc, (e.g. the number of the colour of Magic, octarine) and should never be spoken by wizards especially in certain places. Doing so may allow the ancient dungeon dimension creature "Bel-Shamharoth the sender of eight" to break through (this is taken to a somewhat-extreme end in one book, as even the narrator takes great pains to avoid saying the word). On the other hand, eight turns up in many places one would expect the number seven in our world (e.g. the Discworld week contains eight days, not seven). After The Colour of Magic, both the colour and the number eight no longer appeared as dangerous; this is possibly due to certain actions by Rincewind during the book.
The Disc's magical field is centred on the Cori Celesti. Everyday natural forces, such as light and magnetism, are muffled by the power of the Disc's magical field, and rather than a magnetised needle, navigators on the Disc use a compass with a needle of the magical metal octiron, which will always point towards Cori Celesti. Light is so oddly affected by magic, as it passes into the Disc's atmosphere, it actually slows down from millions to hundreds of miles an hour. One odd effect of this is that the Disc has time zones, when, as a flat world, it shouldn't. Another effect is that, as reported in Thud!, the red- and blue-shifting of light becomes noticeable when travelling at speeds of merely a hundred and twenty miles per hour.
More significantly, it is also belief that gives the gods their powers. Discworld gods start off as tiny spirits, and gain power as they gain believers; this is explored most thoroughly in Small Gods. A similar effect has led to the "reification" of mythological beings symbolising abstract concepts, such as Death, the Hogfather and other Anthropomorphic personifications. In Hogfather, the assassin Mr. Teatime tries to kill the patron of Hogswatch by using an old magic that involves controlling a person with a part of their body (in this case, the teeth collected by the Tooth Fairies), in order to stop children from believing in him, and almost succeeds.
On the Disc, if a story or legend is told often enough and believed by enough people, it becomes true. This is known as the law of narrative causality. Dragons, as Terry Pratchett explains, do not breathe fire because they have asbestos lungs, they breathe fire because that is what dragons do. On the Disc, if a witch goes bad, she will inevitably build a house of gingerbread and lure children to their doom, only to be thrown into her own oven. If a miller has a third son, he will invariably leave him only his cat, and that cat will then, of course, lead the boy onto fame and fortune. A hero will win only when outnumbered. Million-to-one chances to escape certain death are routinely successful and they "crop up nine times out of ten". Witches often employ narrative in their magic, but consider it ethically tricky since it is interfering with free will. This is the source of Granny Weatherwax's hatred of fiction. Knowledge of stories, their use and how to change them forms the basis of many forms of magical power. Lillith and Black Aliss use stories, and Granny Weatherwax uses them and changes them. The habit of many Discworlders to take metaphor literally has combined with the power of belief to produce some very odd areas. The Place Where The Sun Does Not Shine, for instance, is a deep crevasse in Lancre, incidentally located between a rock and a hard place.
The Disc's nature is fundamentally teleological; its basic composition is determined by what it is ultimately meant to be. Its primary element, out of which all others spring, is known as narrativium, the elemental substance of Story. Nothing on the Disc can exist without a Story first existing to mould its destiny and determine its form. This is, perhaps, a take on the fact that nothing can ever happen on the Disc unless it is written in a story by Terry Pratchett (see metafiction).
The Discworld is populated by numerous classic fantasy and mythological races as well as humans. While humans are typically the main inhabitants of the major cities there are many other races that have left their traditional domain and integrated with other, sometimes hostile, species. Pratchett has different characteristics for some of these races when compared to other noted authors.
The Discworld calendar was first defined in a footnote in The Colour of Magic, and has been expanded upon in later novels and The Discworld Almanak (2004). It has numerous oddities, the chief of which is its length.
The calendar is based on a Great Year, or Astronomical Year, defined as the time it takes for the Disc to revolve once on the backs of the elephants. This lasts 800 days and contains two of each season (Midsummer occurs at a given point when the sun passes directly overhead, midwinter when it passes perpendicularly. However most people, especially farmers, consider four seasons to be a year, so an Agricultural Year of 400 days is used for most purposes.
The agricultural year is divided into 13 months:
Each week has eight days: Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday and Octeday.
The 32nd of December, or the day before the New Year, is known as Hogswatchnight. Traditionally associated with pig-killing, to ensure there is enough food for the rest of the winter. Many Hogswatch traditions are parodies of those associated with Christmas, including a decorated oak tree in a pot, strings of paper sausages, and, of course, a visit by the Hogfather. By tradition, witches do not leave the house on Hogswatchnight. No-one knows why, but that's not the point, this is true of a lot of Hogswatch traditions. Nanny Ogg gets around this tradition by inviting the rest of the town in instead.
In the Omnian religion, Hogswatchnight is called the Fast of St Ossory. Omnians celebrate with fasting, prayer meetings, and the exchange of religious pamphlets.
Other calendars count from various other events, and different schools of astronomy give the years different names. The Theocracy of Muntab has a calendar that counts down, rather than up. The reason for this is unknown, but people are very nervous about it reaching zero.
The Post Office, detailed alongside the clacks towers in Going Postal, went through a time of disrepair before Moist von Lipwig turned it into a successful enterprise. The use of mail coaches allows letters to be delivered around the Unnamed Continent, with different cities and organisations having their own set of stamps.
Extensive travel is rare on the Discworld, with many people living in one area for their entire lives. While the city of Ankh Morpork attracts many immigrants, these seldom return home and instead send letters, and possibly money, back to their relatives. Much of the travel that does occur takes place by coach, although services can be somewhat sporadic and unpredictable, especially in less populated areas. Travel by river boat is also known, though travelling the river Ankh usually involves getting out and pushing.
The Disc's magic users and non-human races frequently have their own unique methods of travel. For instance, dwarves have vast underground networks of navigable tunnels with wagonways and canals; gnomes and pictsies can fly on the backs of birds; and banshees and vampires can fly unaided. Witches often fly using broomsticks. These allow skilful operators to fly high enough to clear mountains, and, in one quite-likely unrepeatable event, overtake the night. Magic carpets are not unheard of, and wizards have one of the most spectacular methods of transportation: seven-league boots. However these are mainly spectacular when something goes wrong. It can be tricky using an artifact that relies on you putting one foot 21 miles in front of the other. The groin strain is tremendous.
Wizards can also teleport things, but due to the selective nature of Discworld physics this is very tricky for journeys of more than a short distance. If you do not take into account that what you're moving will end up in a part of the Disc going quite fast in a different direction, and compensate for its desired new movement vector (as well as its mass) upon arrival it may, as Ponder Stibbons might say, "experience a sudden radical reconfiguration of topology and acquire an instantaneously-imposed increase in its level of internal kinetic energy." (Ridcully would describe the result as "being squashed flat while bursting into flames.") With thauma-mathematical aid from Hex, long-range teleportation spells can be made survivable, but they are expensive, unpredictable, and extremely uncomfortable to experience.