A system of writing invented by Dwarves. Moon-letters were ordinary Runes, but written in such a way that they could only be read when the light of the moon shone behind the writing. Some moon-letters could be read by any moonlight, but it was possible to make the writing even more secret. More sophisticated moon-letters could only be read by the same shape of moon, or at the same time of year, as when they were written.
Moon-letters were written with silver pens. The substance Ithildin, made from Mithril, reflected only starlight and moonlight, and it seems likely that ithildin, or something very similar, was used by the Dwarves to write their moon-letters.