The Moirai are daughters of Nyx, the primordial goddess of night, born from the darkness before the gods themselves had shape or name. Some later poets say their father was Zeus and their mother Themis, the Titaness of divine law, but the older tradition holds that they came from Night alone, and that is why they answer to no king. They dwell in a cavern or in the halls of Olympus, and from the first breath of every child they are present, invisible but working. Clotho, whose name means "the Spinner," sits with her distaff and draws the bright thread from the void, twisting it with her fingers into the cord of a new life. The thread is fine or coarse, strong or brittle, and once spun it cannot be unmade. Lachesis, "the Allotter of Lots," takes the thread and measures it against her rod, deciding the span of years and the weight of fortune. A hero might receive a long thread thick with glory, or a short one bright with brief joy. Atropos, "the Inevitable," sits apart with her shears of bronze, and she does not measure or spin. She only waits, and when Lachesis's rod reaches its end, she cuts.

Their power is absolute. When the hero Meleager, the prince who led the great hunt for the Calydonian Boar, a monstrous beast sent by the goddess Artemis to ravage his homeland, was born, the Moirai appeared in his mother's chamber. Althaea, his mother, saw them standing by the fire, and she heard their decree. They said that Meleager would live only as long as a certain log burned upon the hearth. Althaea snatched the log from the flames and hid it in a chest, and her son grew to be a warrior without equal. But years later, in a quarrel over the spoils of the hunt, Meleager killed his own uncles. Althaea, torn between love for her son and duty to her brothers, took the hidden log from the chest and cast it into the fire. As the wood burned, Meleager felt his life drain away, and when the log turned to ash he died, exactly as the Fates had spoken. No prayer, no god, and no hero's strength could undo what the Moirai had spun.

Even the gods themselves acknowledge the sisters. In the Iliad, the great epic of the Trojan War, the hero Achilles, the greatest warrior of the Greeks whose mother Thetis, a sea goddess, had tried to make him immortal, holds the body of his beloved friend Patroclus and weeps. He knows that Patroclus has died because the Fates decreed it, and he knows that his own death will follow soon after Hector, the Trojan prince and champion, falls by his hand. When Zeus, the king of the gods, sees Achilles raging across the battlefield, he weighs the fates of the hero and of Hector in his golden scales. The scale sinks for Hector, and Zeus, who loves the Trojan prince, cannot save him. The Fates have spoken, and even the father of gods must let the thread be cut. In another tale, the goddess Leto, the mother of Apollo and Artemis, swore that no land under the sun would give her refuge to bear her children. But the island of Delos, a floating rock in the sea, was not yet bound by the Fates to be fixed in place. Leto found refuge there, and in gratitude Apollo and Artemis anchored the island with four pillars, but the Fates had already allowed it, for Delos's destiny was to become the most sacred place in the Greek world.

The Moirai also appear in the story of Admetus, the king of Pherae, who was fated to die young. His friend Apollo, the god of music and prophecy who had been forced to serve as a mortal shepherd for killing the Cyclopes, the one-eyed smiths who forged Zeus's thunderbolts, persuaded the Fates to grant a reprieve. They agreed that Admetus could live if someone else would die in his place. His parents refused, his friends refused, but his wife Alcestis, the noblest of women, volunteered. She died so he might live, and for a moment the Fates were outwitted by love. But even then, the thread was not truly unspun. Heracles, the hero forced to complete twelve impossible labours, came to Admetus's house and learned what had happened. He wrestled with Death himself at Alcestis's tomb and brought her back, but the Fates allowed it because her time had not truly ended, or perhaps because even they respect a debt paid in full.

In Roman times, the Moirai were called the Parcae, and their names became Nona, Decuma, and Morta. In Norse mythology, three similar figures called the Norns dwell at the base of the world-tree Yggdrasil, weaving the fates of gods and men. The image of three women controlling life and death has spread across cultures and centuries, from ancient temples to medieval tapestries to modern literature. The Moirai remind every listener that life is not random, that it has a beginning, a middle, and an end, and that the end comes for all, measured and cut with terrible precision.