Letters, Trees and Modern Use

The association of the ogham letters with trees is genuine to the medieval Irish tradition, though not every letter is named for a tree, and it is this arboreal scheme that later writers, Robert Graves above all, developed into the modern "Celtic tree" lore and calendar. The atlas keeps the historical alphabet and its medieval glosses distinct from those twentieth-century constructions.

The entries give each letter its name, sound, and traditional associations. The letters are grouped into sets of five called aicmi, treated in their own category; the modern tree calendar is treated under the revivals.