The fourth ogham family holds all five vowels and is cut in a way that sets them apart at a glance.
The geometry
Aicme Ailme is cut as short strokes crossing the line at a right angle. Its five members carry one to five of them: Ailm, Onn, Ur, Eadhadh and Iodhadh, with the values A, O, U, E and I.
On a standing stone these are often mere notches on the arris that do not run onto either face, which is why the family is sometimes described as notched rather than stroked. That makes it the least durable of the four: the edge weathers first, and a notch has less material behind it than a stroke carried down a face. Inscriptions whose consonants are legible and whose vowel counts are uncertain are a recognised pattern in the corpus.
Square against oblique
The only thing separating this family from the third is the angle of the crossing, square against oblique. Careful editions of ogham inscriptions therefore describe the angle of every crossing character rather than simply transcribing a letter, because on a worn stone that angle is what the reading turns on.
Where the alphabet ends
Iodhadh, five notches, is the twentieth and last letter of the alphabet proper. The additional characters known as the forfeda are composite shapes rather than stroke counts, vary between manuscripts, and are effectively absent from the orthodox epigraphic corpus. A modern chart showing twenty-five ogham characters as one alphabet is merging two different things.