Oiolairë was an evergreen tree, whose leaves remained green in the sea air, even after being cut from the tree itself. The Eldar of Eressëa had a tradition of placing a bough cut from this tree in the prow of their vessels, as a mark of friendship with the Maiar of the Sea. After the foundation of Númenor, the Elves brought both the tree and the tradition to that land, and the custom arose that a Green Bough of oiolairë, usually cut by a woman of the captain's family, should be set aboard a ship before it set out from port. For a ship to leave harbour without a Bough of Return was considered a dangerous omen among the Númenóreans.