Ac has no cognate at all in the twenty four rune Elder Futhark; it is the first of five runes, Ac, Aesc, Yr, Ior and Ear, that the Anglo-Frisian runic tradition added to the row purely to cover sounds of Old English phonology the inherited continental and Scandinavian twenty four signs had no letter for, extending the futhorc to twenty nine runes in its classic form as the surviving rune poem records it.
R. I. Page's account of the futhorc's development treats this expansion as a genuinely practical, linguistically driven change rather than a symbolic or religious one, the Anglo-Saxon scribes and carvers who used runes for real writing, on coins, memorial stones and small portable objects, needing signs for vowel distinctions their spoken language had developed that the older row simply could not represent. Ac's own sound value, a, fills exactly this kind of gap, and its placement as the first of the five additions, immediately after Daeg closes the twenty four inherited runes, keeps the row's overall structure intact, an old, shared core followed by a distinctly Anglo-Saxon extension, rather than interleaving the new signs among the old ones. Page adds that the oak's dual reading, food source and ship timber together in one stanza, makes Ac a useful example of how a single Anglo-Frisian addition could carry as much layered cultural content as any of the twenty four older, inherited runes despite its later date.