The third family of ogham changes the geometry. Where the first two families cut strokes beside the line, the third cuts them through it.
How the letter is formed
Muin opens the third family, Aicme Muine, and is cut as one stroke crossing the line at an angle. A single oblique stroke through the arris is unmistakable at a glance, which is why the third family is the easiest of the four to identify on a weathered stone even when the count is uncertain.
Four geometries, not four positions
The four families are distinguished by how the strokes meet the line rather than by where they sit along it: beside it on one side, beside it on the other, across it obliquely, and across it square. That choice makes the system robust. A stroke that crosses the line cannot be mistaken for one that does not, however the reader is standing, which is precisely the ambiguity that troubles the first two families.
The sound
Muin carries the value M. It appears throughout the surviving corpus of orthodox inscriptions, which are almost all personal names in the genitive case, cut on standing stones in Ireland and in Irish-settled parts of Wales, Scotland and the Isle of Man, and dated on linguistic grounds to roughly the fourth to the sixth centuries.
The vine attached to this letter comes from the manuscript glosses. A vine is not native to Ireland, which is one of several reasons the tree glosses are read as a learned scheme rather than as a record of a living association, and the atlas treats that layer separately.