A third-family ogham letter crosses the line at an angle, and Straif takes four such strokes.
How the letter is formed
Straif is the fourth letter of the third family, Aicme Muine, cut as four parallel strokes crossing the line obliquely.
The value, and why it is written Z
The manuscripts hand this letter to a Latin-trained scribe who needs a Latin character for it, and the character chosen is Z. That choice tells us what the scribes did, not what the letter sounded like. Latin z was itself a borrowed letter with an unstable value in medieval usage, so it functioned as a place to put a sound that the ordinary Latin alphabet had no slot for.
Proposals for the original value cluster around a sibilant distinct from plain S, and the letter's rarity in the surviving inscriptions means the question cannot be closed from the corpus as it stands. The atlas records the manuscript convention as a convention and leaves the earlier value open.
In the corpus
The surviving orthodox inscriptions number a few hundred, are almost entirely personal names in the genitive case, and were 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.
The blackthorn attached to this letter belongs to the manuscript gloss tradition and is recorded separately in this atlas.