The moon was shining in a broad silver crescent. He held up the map and the white light shone through it. "What is this?" she said. "There are moon-letters here, beside the plain runes which say 'five feet high the door and three may walk abreast.' "
"Moon-letters are rune-letters, but you cannot see them," said Elrond, "not when you look straight at them. They can only be seen when the moon shines behind them, and what is more, with the most cunning sort it must be a moon of the same shape and season as the day when they were written. The dwarves invented them and wrote them with silver pens, as your friends could tell you. These must have been written on a midsummer's eve in a cresent moon, a long while ago."