Moon-letters rule

The Moon-letters rule governs a special kind of rune-letters invented by the Dwarves, which are invisible under ordinary viewing conditions. As Elrond explains to Bilbo Baggins, Thorin Oakenshield, and Gandalf at the Last Homely House, these runes "cannot be seen" when looked at directly; they are revealed only when the moon shines behind them. For the more cunning variety, the moon must be of the same shape and season as the day on which the letters were written. The Dwarves wrote them with silver pens, a detail Elrond shares with the company.

Discovery on Thror's Map

The Moon-letters rule becomes crucial during the company's stay at Rivendell. Elrond examines Thror's Map under the light of a broad silver crescent moon and discovers moon-letters beside the plain runes that read "five feet high the door and three may walk abreast." The hidden message, written long ago on a midsummer's eve in a crescent moon, instructs: "Stand by the grey stone when the thrush knocks, and the setting sun with the last light of Durin's Day will shine upon the key-hole." This revelation provides the key to opening the secret door into the Lonely Mountain, a piece of knowledge that had been lost to Thorin and his companions.

Significance for the Quest

The Moon-letters rule directly enables the company to locate and open the secret entrance to the dragon's lair. Without Elrond's ability to read the moon-letters, the dwarves would have had no way to know the precise conditions required to open the door. The rule thus functions as a narrative device that introduces a specific time constraint and a set of observable signs—the thrush's knock and the alignment of sun and moon on Durin's Day—that the characters must wait for and recognize. This transforms the quest from a simple search for a hidden door into a race against the calendar, heightening the tension as autumn wanes and the last moon of the season approaches.