Ur is the second of the sixteen Younger Futhark runes, descending from Uruz, the Elder Futhark's own second rune, its name and general sound value u carried forward across the reduction from twenty four to sixteen signs even where, as the poems themselves show, the specific meaning attached to the name had already begun to drift by the time of the surviving medieval commentary.
The Younger Futhark's sixteen signs had to cover the full vowel and consonant inventory of Old Norse, a language that had itself undergone substantial sound change since the Elder Futhark's original design, so Ur, like several of the row's other vowel signs, came to represent more than one distinct sound in practice, an underspecification runologists treat as a defining structural feature of the whole Younger Futhark system rather than a flaw, since context and a reader's own knowledge of the language filled in what the limited sign inventory left ambiguous. Michael Barnes' handbook treats this deliberate economy, fewer signs than the language strictly needed, as a genuine puzzle in runic scholarship, since the reduction happened even as the Old Norse sound system was if anything growing more complex, the opposite of what a simple wish for clearer writing would predict.