Ogham encodes each letter as a family plus a count of strokes from one to five.
How the letter is formed
Coll is the fourth letter of the second family, Aicme hUatha, and is cut as four strokes on that family's side of the line.
Two letters for one modern sound
Coll carries the value C, and the letter that follows it, Quert, carried a distinct sound in the language of the inscriptions. In Primitive Irish the two were different phonemes. By the Old Irish of the manuscripts they had fallen together, leaving Quert with no distinct value and Coll doing the work of both.
That merger is visible in the inscriptions themselves. It is one of the clearest cases where the ogham corpus is evidence for the history of the Irish language rather than merely a curiosity of writing, because the letters record a distinction the later language no longer made and the manuscripts could no longer explain.
In the corpus
The surviving orthodox inscriptions are almost entirely personal names in the genitive case, cut on standing stones in Ireland and in Irish-settled parts of Britain, dated on linguistic grounds to roughly the fourth to the sixth centuries. Coll is common in them.
The hazel attached to this letter belongs to the manuscript glosses, and the calendar schemes in modern circulation are later still. The atlas keeps the three layers apart.