Celtic Ogham

Ogham was a system of writing which used horizontal or slanting notches cut on stone or wood to indicate letters. Existing examples suggest that Ogham was used primarily on grave and boundary markers. Evidence exists however, to support its use by Druids for recording tales, histories, poetry, genealogies, and the like. In addition, each letter was named for a particular tree, and a vast lore of religious and mythological knowledge could be encoded in cryptic verse. The names of the main twenty letters are the names of 20 trees sacred to the druids.

There is no direct evidence that the Ogham alphabet was used [in antiquity] for divination or any other magical purposes. It has twenty letters, grouped into four 'aicme' of five letters each. A fifth Aicme (consisting of another five letters) was invented later to account for sounds in Latin that do not occur in Gaelic.