The ogham alphabet is organised into four aicmi, families or groups, of five letters each, and each group takes its name from its first letter. Aicme Beithe is the first, named for beith, and its five letters are beith, luis, fern, sail and nin, giving the sounds b, l, f or v, s and n. In the script the letters of this group are cut as one to five strokes to the right of the stemline, or below it when the line is horizontal. Ogham is attested in some four hundred surviving stone inscriptions, chiefly in Ireland and in Welsh and Scottish areas of Irish settlement, dating from roughly the fourth to the seventh centuries, and the great majority are simple memorial formulae giving a personal name in the genitive.
Beithe's own name is one of the more secure cases in a system otherwise full of dispute: modern scholarship, chiefly Damian McManus's A Guide to Ogam (1991), accepts beithe as a genuine, unambiguous tree name, birch, one of only about five of the twenty ogham letter names that etymology and the medieval glosses agree on without real controversy. That relative certainty makes Aicme Beithe an unusual case to open the alphabet with, since the letter-name debates that affect several of its sibling groups, addressed in the articles on Aicme hUatha and Aicme Muine, do not apply to its own founding letter in anything like the same degree.