Ogham writing is a way of recording language designed to be written on stone, by cutting straight lines on edges, or imitated edges. Surviving inscriptions are mostly written in (Old) Irish, for which the script seems to have been developed, and Pictish. In recent times, it has occasionally been used for a simple cypher, and rather more often for certain mystical uses, by druids, 'Celts' and other pagans. The equivalent in Ogham of the 'alphabet' in Roman writing, the standard script in West European languages, is the Beith-Luis-Nion.
The Ogham system works on a simple plan, which can be used horizontally or vertically. Each one of the basic letters (feda in Erse), of which there were originally twenty, grouped into four 'families' (aicmi), is formed by a single stroke formed on a central 'spine', or foundation-line [druim]. (The spine is sometimes the edge of a stone block on which the letters are formed; on a flat surface, it is drawn as a straight line.) These strokes are horizontal when the spine (and consequently the message) is vertical, and to the right or left, or both; when the spine is horizontal, the strokes that form the letters are up, down, or fully crossing.
- The first family (Aicme Beithe, or 'family of the birch tree [representing the letter 'B']) contains letters with a stroke only to the right of, or down from, (but not crossing) the spine contains. These are: with a single stroke, the letter representing the sound of 'B'; with two strokes, 'L'; with three strokes, 'V'; with four, 'S'; and with five, 'N'.
- The second family (Aicme hÚatha, or 'family of the whitethorn [usually given as representing the letter 'H', but this is not certain]) contains letters with a stroke only to the left of, or up from, (but not crossing) the spine. This family contains, with a single stroke, the letter representing the sound (probably) of 'H'; with two strokes, 'D'; with three strokes, 'T'; with four, 'C'; and with five, 'Q'.
- The third family (Aicme Muine, or 'family of the vine [representing the letter 'M']) contains letters with a stroke only crossing the spine diagonally. A single stroke gives the letter representing the sound of 'M'; two strokes, 'G'; three strokes, 'NG'; four, 'Z'; and five, 'R'.
- The fourth family (Aicme Ailme, or 'family of the pine tree [representing the letter 'A']) contains letters with a stroke crossing the spine at right angles. It contains, with a single stroke, the letter representing the sound of 'A'; with two strokes, 'O'; with three strokes, 'U'; with four, 'E'; and with five, 'I'.
After these basic twenty letters (feda), supplementary letters (forfeda) were developed. There is one family, whose letters have widely varying values to meet the different needs of the scribes at different times and in different places. The five letters are not always consistently written or interpreted. The first is always a diagonal cross placed saltire-wise on the spine, representing variously the consonant 'K' or the diphthong 'EA'. Second is a diamond, or rhombus (having similar triangles to the previous, but disposed on opposite sides of the spine), representing 'OI'. The third is 'the elbow', a hook with straight lines to the right of, or below, the spine, representing 'UI' or 'U'; the fourth is two saltire crosses laid gridwise (not quite superimposed) to the right, or below, for 'P', 'IA' or 'IO'. Finally, a grid of four x four lines, on one side of the spine (which side varies) represents the diphthong AE, or the consonants CH or X.
For ease of use, four other characters can be seen. A stylized feather, looking like a Roman 'V', is placed on the spine at either end of the text. Its open end is away from the writing to be considered: the apex points towards the text. A section of the spine left blank, with no letters, corresponds exactly to a space in our writing. A short line, to the left of, or above, the spine and parallel to it, is the sign for the letter 'P'. This was needed, as the old ogham 'P' (the fourth of the furfeda) had been adapted for vowels, as the phoneme 'P' had ceased to be used in Erse. There was no 'P' until [[Latin texts necessary in the Church began to reinstate it in such borrowings as the name of Ireland's own [[saint], Saint Patrick.