The forfeda

A fifth group, the forfeda or supplementary letters, was added later to write sounds, chiefly diphthongs and non-native sounds, that the original twenty letters could not represent. These are more elaborate in form and appear mainly in the manuscript tradition rather than in the early stone inscriptions.

The atlas treats each aicme as a unit, giving its member letters and their shared placement, alongside the individual letters in their own category. The wider ogham system, and the medieval tree-lore attached to it, are treated under the ogham and letter categories.