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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 needle of octiron, which will always point towards Cori Celesti, in a compass. Light is so oddly affected by magic that, as it passes into the Disc's atmosphere, it actually slows down from millions to hundreds of miles an hour, so that, as the Disc has no horizon, it is actually possible to see days into the past from some of the higher mountain peaks. 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 traveling at speeds of a hundred and twenty miles per hour.

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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 fourth 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. It is the source of Granny Weatherwax's hatred of fiction.

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