Iodhadh is the twentieth and last letter of the ogham alphabet proper, and it takes the maximum count in the last family.
How the letter is formed
Iodhadh is the fifth letter of the fourth family, Aicme Ailme, and is cut as five short strokes crossing the line at a right angle.
Where the alphabet stops
Four families of five letters is twenty, and twenty is the whole of the epigraphic alphabet. Nothing in the geometry prevents a sixth stroke, and no inscription uses one. The limit is a reader's limit rather than a scribe's: five parallel marks can be counted at a glance and six cannot, particularly on an eroded arris where a stroke may be missing.
When the manuscript tradition needed further characters it therefore did not extend the count. It introduced the forfeda, a fifth group of composite shapes that are inconsistent between manuscripts and effectively absent from the stones.
The sound
Iodhadh carries the value I. It appears throughout the surviving orthodox corpus, which is made up almost entirely of 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, dated on linguistic grounds to roughly the fourth to the sixth centuries.
The layers afterwards
The yew attached to this letter comes from the medieval manuscript glosses. The calendrical and divinatory schemes now in wide circulation descend from twentieth-century work and are later still. The atlas records all three separately.