Ogham is the earliest form of writing in the Irish language, an alphabet of some twenty letters (later extended by a further five) in which each character is formed of one to five straight or angled strokes cut across, or on either side of, a central line, a "stem" that on standing stones is usually the vertical edge of the stone itself, read from the bottom up.
Several hundred ogham stones survive, chiefly in Ireland and in areas of Irish settlement in Wales, Scotland, and the Isle of Man, dating mostly from roughly the fourth to sixth centuries CE; most are memorial inscriptions giving a personal name and lineage, and they are among the earliest written records of the Irish language and important evidence for the transition from paganism to Christianity in Ireland. The letters bear the names of trees and plants in the later manuscript tradition (birch, rowan, alder, and so on), which gave rise to the modern notion of a "tree alphabet." Medieval Irish scholars treated ogham with great learning, the Auraicept na n-Eces ("Scholars' Primer") and the Book of Ballymote preserve elaborate systems and variant alphabets, and reported its use as a cipher and a mnemonic. In the modern era ogham has been widely adopted within Celtic-inspired Neo-Pagan and Druidic practice as a system of divination and correspondence, drawing on the tree-names and their symbolism. It is thus both a genuine ancient script of real historical and linguistic importance and, in its modern revival, a divinatory and magical alphabet.