Ogham identifies a letter by its family and by a stroke count from one to five.
How the letter is formed
Uath opens the second family, Aicme hUatha, and is cut as one stroke on the second family's side of the line, the side opposite the first family. In the horizontal manuscript layout the second family is written above the line where the first family sits below it.
A letter with almost nothing to do
Uath is given the value H, and that is where the difficulty begins. Old Irish had no phoneme corresponding to an independent h, and the letter is correspondingly rare in the surviving inscriptions. A letter that encodes a sound the language does not use is unusual in an alphabet designed for that language, and it is one of the reasons scholars have argued that the ogham order was constructed on the model of a Latin alphabet rather than assembled from Irish phonology alone.
That argument is live rather than settled, and the atlas records it as a scholarly position rather than as a conclusion.
In the corpus
The surviving orthodox inscriptions are almost all personal names in the genitive, cut on standing stones in Ireland and in Irish-settled parts of Wales, Scotland and the Isle of Man, dated on linguistic grounds to roughly the fourth to the sixth centuries. Uath appears in very few of them.
The hawthorn attached to this letter belongs to the later manuscript glosses and is recorded separately in this atlas.