ogham (Modern Irish), ogam, ogum (Old Irish), oghum (ScottishGaelic):

The earliest form of writing in Irish in which the Latin alphabet is adapted to a series of twenty 'letters' of straight lines and notches carved on the edge of a piece of stone or wood. Letters are divided into four categories of five sounds:

(see ogham table )

A twenty-first symbol, an upturned arrow, was used for the letter p in British inscriptions. Notches and grooves appear on one or both sides of a foundation-line [druim]. Each letter was named for a different tree, e.g. a = ailm [pine], b = beithe [birch], etc.,

Designations for the letters q, v, and z, which are not used in Irish, support the now widely accepted interpretation of ogham as an expression of Irish through the Latin alphabet. The current view displaces many colourful speculations on ogham's origin: runic alphabet of Scandinavia, Chalcidic Creek, northern Etruscan, etc.

Ogham inscriptions date primarily from the 4th to 5th centuries and are found mainly on standing stones; evidence for inscriptions in wood exists, but examples do not survive. The greatest concentration of surviving ogham inscriptions is in southern Ireland; a 1945 survey found 121 in Kerry and 81 in Co. Cork, while others are scattered throughout Ireland, Great Britain, and the Isle of Man, with five in Cornwall, about thirty in Scotland, mainly in 'Pictish' areas, and more than forty in Wales. South Wales was an area of extensive settlement from southern Ireland, including the migration of the De'isi. Ogham was also used for Pictish. In Wales, ogham inscriptions have both Irish and Brythonic-Latin adjacent inscriptions.

Most ogham inscriptions are very short, usually consisting of a name and a patronymic in the genitive case. They are of linguistic rather than literary interest, because they show an older state of the Irish language than found in any other written sources. Many appear to be memorials to the dead, while others mark the border between two lands. Although the knowledge of ogham was never lost to scholars (at least one 19th-cent. grave-marker uses it), the notion that ogham was employed for occult or magical purposes dogs critical commentary. As late as the 1930s the eminent archaeologist R. A. S. Macalister proposed that ogham was part of the secret language of 'druidic freemasonry'.

Seán O'Boyle suggested (1980) that the key to explaining ogham is harp notation. The god of rhetoric and eloquence, Ogma, is an attributed creator; his name and the word appear to be philologically related.