Eadhadh is the fourth of the five vowels, and like the rest of its family it is cut square across the line.
How the letter is formed
Eadhadh is the fourth letter of the fourth family, Aicme Ailme, and is cut as four short strokes crossing the line at a right angle.
Twenty letters, and then the problem of more
Eadhadh sits near the end of the original twenty. Those twenty are what the stones use, and they are enough for the language of the inscriptions. The manuscript tradition, working with Latin loanwords and with sounds the older orthography had no character for, wanted more, and added a fifth group called the forfeda.
The forfeda are unlike everything before them. They are composite shapes rather than counts of strokes, they vary between manuscripts, and they are effectively absent from the orthodox epigraphic corpus. They belong to a scholarly tradition writing about ogham rather than to the practice of cutting it, which is a distinction worth holding on to when reading any modern chart that presents twenty-five ogham characters as a single alphabet.
The sound
Eadhadh carries the value E. The surviving orthodox inscriptions are almost all personal names in the genitive case, cut in Ireland and in Irish-settled parts of Britain and dated on linguistic grounds to roughly the fourth to the sixth centuries.
The aspen attached to this letter is a manuscript gloss and is recorded separately in this atlas.