They live beneath the roots of Yggdrasil, (although some accounts have it that they dwell above the arch of the Bifrost Bridge), where they weave the tapestry of fates. Each man's (and woman's) life is a string in their loom, and the length of the string is the length of the person's life.
Thus everything is preordained in the Norse Religion: even the gods have their own threads, though the norns do not let the gods see those. This clear subjection of the gods to a power outside their control and the implication that they, too, will have an End are major themes of the literature surrounding the mythology.
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Norns are also an artificial lifeform in the computer game Creatures.