Editing Sourcery (section)

==Premise==
On the [[Discworld (world)|Discworld]], "sourcerers"—wizards who are sources of magic, and thus immensely more powerful than normal wizards—were the main cause of the Great Mage Wars that left areas of the Disc uninhabitable. As eight is a powerful magical number on Discworld, men born as the eighth son of an eighth son are commonly wizards. Since sourcerers are born the eighth son of an eighth son ''of an eighth son'', they are "wizards squared". To prevent the creation of sourcerers, therefore, wizards are not allowed to marry or have children.

There are many references to ''[[geis|geasa]]'' in the text, which are idiosyncratic curses, vows or obligations placed upon a person (usually a hero, such as [[Cúchulainn]] in [[Irish mythology]]).