The Prose Edda 1.15

Then said Gangleri: "Where is the chief abode or holy place of the gods?" Hárr answered: 'That is at the Ash of Yggdrasill; there the gods must give judgment everyday." Then Gangleri asked: "What is to be said concerning that place?" Then said Jafnhárr: "The Ash is greatest of all trees and best: its limbs spread out over all the world and stand above heaven. Three roots of the tree uphold it and stand exceeding broad: one is among the Æsir; another among the Rime-Giants, in that place where aforetime was the Yawning Void; the third stands over Niflheim, and under that root is Hvergelmir, and Nídhöggr gnaws the root from below. But under that root which turns toward the Rime-Giants is Mímir's Well, wherein wisdom and understanding are stored; and he is called Mímir, who keeps the well. He is full of ancient lore, since he drinks of the well from the Gjallar-Horn. Thither came Allfather and craved one drink of the well; but he got it not until he had laid his eye in pledge. So says Völuspá:

All know I, Odin, where the eye thou hiddest, In the wide-renowned well of Mímir; Mímir drinks mead every morning From Valfather's wage. Wit ye yet, or what?

The third root of the Ash stands in heaven; and under that root is the well which is very holy, that is called the Well of Urdr; there the gods hold their tribunal. Each day the Æsir ride thither up over Bifröst, which is also called the Æsir's Bridge. These are the names of the Æsir's steeds: Sleipnir is best, which Odin has; he has eight feet. The second is Gladr, the third Gyllir, the fourth Glenr, the fifth Skeidbrimir, the sixth Silfrintoppr, the seventh Sinir, the eighth Gísl, the ninth Falhófnir, the tenth Gulltoppr, the eleventh Léttfeti. Baldr's horse was burnt with him; and Thor walks to the judgment, and wades those rivers which are called thus:

Körmt and Örmt and the Kerlaugs twain, Them shall Thor wade Every day when he goes to doom At Ash Yggdrasill; For the Æsir's Bridge burns all with flame, And the holy waters howl."

Then said Gangleri: "Does fire burn over Bifröst?" Hárr replied: "That which thou seest to be red in the bow is burning fire; the Hill-Giants might go up to heaven, if passage on Bifröst were open to all those who would cross. There are many fair places in heaven, and over everything there a godlike watch is kept. A hall stands there, fair, under the ash by the well, and out of that hall come three maids, who are called thus: Urdr, Verdandi, Skuld; these maids determine the period of men's lives: we call them Norns; but there are many norns: those who come to each child that is born, to appoint his life; these are of the race of the gods, but the second are of the Elf-people, and the third are of the kindred of the dwarves, as it is said here:

Most sundered in birth I say the Norns are; They claim no common kin: Some are of Æsir-kin, some are of Elf-kind, Some are Dvalinn's daughters."

Then said Gangleri: "If the Norns determine the weirds of men, then they apportion exceeding unevenly, seeing that some have a pleasant and luxurious life, but others have little worldly goods or fame; some have long life, others short." Hárr said: "Good norns and of honorable race appoint good life; but those men that suffer evil fortunes are governed by evil norns."