Ur is a vowel, and in ogham the vowels have a geometry of their own.
How the letter is formed
Ur is the third letter of the fourth family, Aicme Ailme, and is cut as three short strokes crossing the line at a right angle.
Square against oblique
The only difference between the third family and the fourth is the angle at which the strokes meet the line: oblique for the consonants of Aicme Muine, square for the vowels of Aicme Ailme. Three oblique strokes are nGetal; three square ones are Ur. On a clean stone the distinction is obvious. On a worn one it is the commonest source of dispute in the epigraphy after stroke counts themselves, and it is the reason careful editions of ogham inscriptions describe the angle of every crossing character rather than simply transcribing letters.
The sound
Ur carries the value U. It appears throughout the surviving corpus, which consists almost entirely of personal names in the genitive case, cut on standing stones in Ireland and in Irish-settled parts of Britain, and dated on linguistic grounds to roughly the fourth to the sixth centuries.
The gloss
The heather attached to this letter belongs to the manuscript tradition, centuries later than the stones. This atlas keeps the epigraphic letter, the medieval gloss and the modern calendrical schemes as three separate records rather than merging them into one.