The Tree Ogham comes from the Celtic Culture. Ogham meaning alphabet.
The exact orgins and knowledge regarding the Tree Ogham is unclear, What is known was passed by word of mouth, until some writings appeared in the Twelfth Century – and then later in 1391, “The Book of Ballymote” and “The Book of Lecan”, 1416.
There is no direct evidence to tell us how it was used. However it seems clear that, at least at some level, it was used a sign language, and obviously an alphabet.
The Druid, the learned class of the Celtic Culture, left no written scripts. Something like the Tree Ogham would have worked as an aide de memoire.
The 20 Trees of The Tree Ogham according to the order given in “The Book of Ballymote”