The Cirth is a semi‑artificial script, based on real‑life runic alphabets, one of several scripts invented by J. R. R. Tolkien for the constructed languages he devised and used in his works. Cirth is written with a capital letter when referring to the writing system; the letters themselves can be called cirth.

A rune is a letter in a set of related alphabets known as runic alphabets native to the Germanic peoples. Runes were used to write Germanic languages before they adopted the Latin alphabet, and for specialised purposes thereafter. In addition to representing a sound value, runes can be used to represent the concepts after which they are named (ideographs). Scholars refer to instances of the latter as Begriffsrunen. The Scandinavian variants are also known as fuþark, or futhark, these names derived from the first six letters of the script, ⟨ᚠ⟩, ⟨ᚢ⟩, ⟨ᚦ⟩, ⟨ᚨ⟩/⟨ᚬ⟩, ⟨ᚱ⟩, and ⟨ᚲ⟩/⟨ᚴ⟩, corresponding to the Latin letters ⟨f⟩, ⟨u⟩, ⟨þ⟩/⟨th⟩, ⟨a⟩, ⟨r⟩, and ⟨k⟩. The Anglo-Saxon variant is futhorc, or fuþorc, due to changes in Old English of the sounds represented by the fourth letter, ⟨ᚨ⟩/⟨ᚩ⟩.