Odin hung on the world tree for nine nights to gain knowledge, especially the runes and their power.
The episode is told in Hávamál, the “Sayings of the High One,” a poem preserved in the Poetic Edda. Odin describes sacrificing himself to himself: he hangs on a windswept tree, is wounded by a spear, receives neither food nor drink, and looks downward into the depths. After the ordeal, he seizes the runes and falls from the tree with new wisdom.
The poem calls the tree a “windy tree” rather than explicitly naming Yggdrasil. The identification with Yggdrasil is the standard interpretation because Yggdrasil is the central world tree and its name is traditionally understood as “Odin’s horse,” an image linked to hanging. Thus, nine is the clear canonical answer, even though the precise tree name is an interpretive connection.
The story reflects a recurring Norse theme: wisdom is costly and may require danger, sacrifice, or suffering.