Elrond reveals that Thorin's map has a secret message, written in moon letters, a type of rune-letters that only can be seen with "a moon of the same shape and season as the day when they were written" (50). Moon letters were invented by the dwarves who wrote them with special silver pens. Elrond interprets the message for Gandalf and Thorin, who are completely surprised, and a little "vexed," that Elrond discovered this secret to the map. The mysterious moon letters tell of how to access a secret door to the Lonely Mountain; the keyhole will only reveal itself on Durin's Day. Elrond's important discovery will play a major role in aiding the dwarves and Bilbo's entry into the Lonely Mountain.