Fate, in Norse cosmology, requires daily maintenance.
Völuspá stanza 20 places three maidens by the well beneath the ash — Urðr, Verðandi and Skuld — who cut lots, carve on wood, and lay down law and choose lives for the children of men. Snorri, quoting it in Gylfaginning, adds the part that changes the picture: the norns who dwell by Urðr's well take water from it every day, together with the clay that lies about the well, and pour it over the ash so that its limbs will not wither or rot. The water is so holy that everything coming into it turns as white as the film inside an eggshell.
The world tree is being eaten from below by Níðhöggr and browsed from above by harts. The reason Yggdrasil still stands is that three women go out every morning with buckets.
The names, and a very old mistake
Urðr is the past — the same word as Old English wyrd, which gives modern English weird, and originally meant "that which has happened." Verðandi is a present participle: "becoming," what is turning. Skuld is from the verb skulu, "shall" or "ought" — obligation as much as futurity; the same root gives English should and the Norse word for a debt.
Past, present and future is the standard gloss and it is not quite right. Urðr is not "the past" in the abstract; it is the accumulated done thing, the record. Skuld is not "the future"; it is what is owed. Norse fate is closer to an account than a timeline.
It is also probably a poetic construction rather than an inherited triad. Urðr is the deep name — the well is called Urðarbrunnr, the well of Urðr, and the word appears independently across Germanic — and the other two look like a poet's elaboration on the grammar of the first. Nor should they be assimilated to the Greek Moirai or the Roman Parcae. Nothing in Old Norse gives the norns a thread, a spindle, or a pair of shears. The spinning and thread-cutting imagery attached to them in almost every modern retelling comes from classical mythology, not Norse. What the norns do with wood is carve, and what they do with lots is cut them.
There are a great many of them
Fáfnismál is explicit, and it demolishes the tidy three. Sigurðr asks the dying dragon who the norns are that come to women in labour, and Fáfnir answers that they are of many kindreds and not of one race: some are of the Æsir, some of the elves, some are the daughters of Dvalinn among the dwarfs.
Snorri repeats this and draws the obvious conclusion — good norns of good descent shape good lives, and where a person has misfortune, ill norns are responsible. This is fate as a distributed system with variable staff quality, which is a strange idea and a memorable one.
Nornagests þáttr dramatises it. Travelling seeresses — called völur, and functionally norns — visit the house at the birth of a boy. Two prophesy well. The third, jostled from her seat and angered, declares the child will live no longer than the candle burning beside him. The eldest immediately snuffs the candle and gives it to the mother, and Nornagestr carries it, unlit, for three hundred years, until he is baptised and lights it himself. The story is late and Christian in its frame, and it is the fullest surviving picture of what the Norse thought a norn at a birth actually did.
The well and the court
Urðr's well is not only a source of water. Grímnismál and Gylfaginning both place a court there: the gods ride over Bifröst to their judgement-seat by the well each day. Thor cannot use the bridge and wades four rivers to get there.
So the tree's root, the well, the fate-women and the assembly of the gods are all at one location. The Norse image of destiny is a þing — a legal assembly — held daily at a spring, with the case law of what has already happened sitting in the water. That the norns "lay down law" in Völuspá is not metaphorical decoration; the poem uses the vocabulary of legislation.
Fate and freedom, without a contradiction
Norse literature is entirely comfortable with characters who know their fate and act anyway, and it does not treat this as a paradox. The sagas are full of the formula that no one can escape their fate — and equally full of people making choices that are treated as real, praiseworthy or contemptible. The virtue on offer is not evading the outcome but meeting it well. Odin knows the wolf will kill him and spends the entire mythology preparing regardless; see Ragnarök for what preparation is worth.
From urd to weird
The norns had a long folk life. Scandinavian and German folklore kept the fate-women at the cradle, and the motif runs straight into the uninvited fairy at the christening in Perrault and the Grimms. Wagner opened Götterdämmerung with three norns whose rope of destiny snaps, which is beautiful and is his own invention. Modern usage keeps urð and wyrd alive in English as "weird," having travelled from "what has happened" through Shakespeare's Weird Sisters to "strange."
For the neighbouring class of female powers, see the valkyries; for the tree they keep alive, Yggdrasil; and for how the whole system fits together, the Norse overview and the family tree, which draws no descent for them because none is recorded.
If the norns assign every life, what is Odin doing consulting seeresses?
Learning, not negotiating. Nothing in the corpus shows any god petitioning the norns or attempting to alter an assignment, and Odin's methods — waking the dead, questioning giants, hanging on the tree — are all techniques of acquisition. The distinction is worth holding onto, because it is what separates Norse fate from Greek: no one bargains with the norns, and no one is punished for trying, because nobody tries.
Is Skuld the norn the same as Skuld the valkyrie?
Völuspá lists a Skuld among the valkyries who ride to choose the slain, a few stanzas after naming Skuld as the third norn. Snorri also mentions a Skuld as the youngest norn and as a valkyrie. Whether one figure holds both offices, or the poet used a common noun meaning "what is owed" twice, cannot be determined — and the ambiguity is itself informative, since it shows how little separated the power that assigns a life from the power that ends one.