In Norse mythology, the Norns (pronounced like “norms” with an “n” instead of the “m”; Old Norse Nornir) are three female divine beings who have more influence over the course of destiny than any other beings in the cosmos. They dwell within the Well of Urd beneath Yggdrasil, the great ash tree that stands at the center of the universe and holds the Nine Worlds in its branches and roots. They shape destiny by carving runes into the trunk of the tree, or, in some sagas and poems, by weaving destiny like a web or tapestry.
Their names are Urd (Old Norse Urðr, “What Once Was”), Verdandi (Old Norse Verðandi, “What Is Coming into Being”) and Skuld (Old Norse Skuld, “What Shall Be”). A common misconception is that they correspond to the past, present, and future in a linear conception of time. A more sensitive analysis shows that they correspond instead to past, present, and necessity in a cyclical conception of time, as is discussed here.
Another common misconception is that the destiny woven or carved by the Norns is final and unalterable, as in the Greek concept of fate. The Norse/Germanic model of destiny, however, is far more dynamic and volatile than this, and leaves ample room for individual agency in the shaping of destiny. While there may be only three Norns with a capital “N,” there are countless norns with a lowercase “n” – norn is an Old Norse word for a generic practitioner of magic. A fuller discussion of this view of destiny can be found here.
If you enjoyed this article, check out my book on the worldview at the heart of Norse mythology, The Love of Destiny: The Sacred and the Profane in Germanic Polytheism.
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 Bauschatz, Paul C. 1982. The Well and the Tree: World and Time in Early Germanic Culture.
 Heide, Eldar. 2006. Spinning Seiðr. In Old Norse Religion in Long-Term Perspectives: Origins, Changes and Interactions. Edited by Anders Andrén, Kristina Jennbert, and Catharina Raudvere. p. 166.