The four ogham families are told apart by the angle at which their strokes meet the writing line, not by where they sit along it.
The geometry
Aicme Muine is cut as strokes crossing the line at an angle. Its five members carry one to five oblique strokes: Muin, Gort, nGetal, Straif and Ruis, with the values M, G, NG, a sibilant written Z in the manuscripts, and R.
Because these strokes cross the line rather than sitting beside it, they cannot be confused with the first two families however the reader is standing. That is the practical advantage of changing the geometry rather than simply moving the strokes further along.
The hardest family to cut and to read
On a standing stone every one of these strokes has to be carried across the arris and down both faces at a consistent angle. Execution varies more in this family than in the first two, and a five-stroke oblique character such as Ruis is the single most difficult letter in the system to read on a weathered stone. A lost stroke turns one valid letter into another valid letter, since there is no redundancy anywhere in ogham.
Two letters with unsettled values
nGetal and Straif are both rare in the surviving inscriptions and both have values that the manuscripts supply rather than the corpus confirming. The manuscript values are recorded here as manuscript values, and the earlier ones are treated as open.