The Groups of the Ogham Alphabet

The letters of the ogham alphabet are arranged in groups of five called aicmi (singular aicme), each named after its first letter. The classical alphabet has four aicmi of five letters each (the aicme of Beithe, of Huatha, of Muine and of Ailme), making twenty letters, cut respectively below, above, across and through the stem-line.

The grouping is not merely alphabetical order but a structural feature of the script: the four aicmi correspond to the four ways a stroke can be placed relative to the line, so a reader can tell a letter's group at a glance from how its strokes sit.