In its earliest form the Ogham alphabet consists of four groups of five characters, giving a total of twenty characters representing the sounds of the Irish language of the period (orthodox Ogham). The characters are made up of between one and five rectilinear lines or scores relative to a stemline, generally vertical on stone using its natural angle or edge, with two of the groups distinguished by the orientation of the scores. The third group transverses the stemline diagonally and the fourth group consists of one to five scores/notches on the stemline as follows:
A fifth group of five 'supplementary characters' or forfeda is included in the manuscript tradition on Ogham. However, only the first of these occurs on the orthodox inscriptions and appears as two diagonal scores crossing each other on the stemline as follows:
This character is used with two values: consonantal /k/ or /x/ (usually transliterated K) and vocalic /e/. In the former usage it is most commonly found in the formula word KOI 'here' (corresponding to HIC IACIT on British inscriptions).
Each of the characters were assigned names, which were meaningful words in the language, although some have since fallen out of use. These names are our most important source of information on the primary values of the characters. McManus (1991, 3) gives the following forms, normalised to Old Irish, from the manuscript tradition:
Beithe, Luis, Fern, Sail, Nin
hÚath, Dair, Tinne, Coll, Cert
Muin, Gort, (n)Gétal, Straif, Ruis
Ailm, Onn, Úr, Edad, Idad
The groups are known as aicmi (pl. of aicme 'family, class, group') in Irish and are named after the initial character giving Aicme Beithe, Aicme hÚatha, Aicme Muine and Aicme Ailme. The characters or letters are generally termed feda (pl. of fid 'wood, tree'), the stemline druim 'ridge, edge, back' and a single score is called flesc 'a twig'. Although a number of the Ogham characters were named after trees, it is no longer accepted that all were. The following is taken from McManus' list of letter names (1991, 36-9; 1988, 127-68):
Among the numerous theories on the origins of Ogham are the following:
Many scholars view Ogham as a cipher to an alphabet (usually Latin) rather than as an alphabet itself. However, there are many features of Ogham which cannot be explained by a simple encoding of the letters of another language, such as Latin. The likelihood is that Ogham was created in Ireland by inventors familiar with Latin grammar to represent the sounds of primitive Irish. Although the manuscript record of the Ogham characters is invaluable in many respects, we must keep in mind that the manuscript tradition is too far removed from the period of the orthodox Ogham inscriptions to be completely reliable. It can be argued, for example, that the characters H, NG and Z, sounds which, to our knowledge, did not exist in initial position in primitive Irish, are 'cosmetic and Latin-based and were chosen on the basis of the contemporary forms of their letter names' (McManus 1991, 33-4). Unfortunately, these three characters are so poorly attested that it is impossible to say with any certainty what the original values may have been.