Ogham

Although strongly associated, indeed often referred to as the "Celtic Tree Alphabet", Ogham is a distinct medieval tradition that uses the names of trees to denote its individual letters in a written alphabet. Since Ogham of itself covers neither the mythology nor botanical lore of trees, it is better understood as an 'add-on' to the subject of the tree alphabet, and not as an explanation or umbrella to it.

The Irish og-úaim ('point-seam'), refers to the ridge or groove made by the point of a sharp weapon, which explains the technique, however it is traditionally credited as having been invented by Ogma, the patron of poetry and eloquence, and one of the three key gods of the Tuatha de Danaan (along with Lug and the Dagda). Ogma's first message was written as seven b's on a birch tree, a warning to Lug Lamfhota that "your wife will be carried away seven times to the otherworld unless the birch protects her". Strictly therefore, the word ogham refers only to the form of letters or script, while the letters themselves are known collectively as the Beith-luis-nin after the letter names of the first letters (as 'alphabet' refers to Alpha and Beta in the greek alphabet). Because the word nin (as opposed to nuin) literally means 'a forked branch', Beith-luis-nin can be assumed to mean the 'forked-branch' style of written letters, or simply the 'Beith-luis letters'. The ogham alphabet originally consisted of twenty distinct characters (feda), arranged in four groups or aicmí (families). Each aicme was named after its first character (Beithe, Huath, Muin, Ailm)