The moon letters brought out additional information. Elrond found them while looking at the map with Gandalf and Thorin one evening. They provided information on the secret passage.
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They were wrote by dwarves but you cannot see them
-T:Prophesied what? -Prophecied when the keyhole to the secret entrance of Lonely Mountain would be visible. -T:Well the moon runes on Thráin's map told them. Also, that's not a prohecy because they made the door to be that way themselves.
The book by J.J.R. Tolken shows Bilbo and Thorin and Co. using a key, Bilbo's sword, Sting, and, of course, the ring. One other thing that is sort-of apart from the map is the fact that the all-important moon-letters, which were found by Elrond the elf.
Durin's Day indicates that the dwarves can open the door with the key on the last moon of Autumn.
A hobbit barrow is referred to as a 'smail.'