Ogham is a writing system, still found on stone monoliths throughout Ireland, Scotland, Wales and parts of England, used by the Druids and poets/Bards of the ancient Celts. It was a message system, a recording system of important deeds and names, a symbolic device used for learning and remembering ancient lore, and a spiritual tool. Each sigil represented a letter or sound and was linked to a tree or plant, with a corresponding "kenning"or poetic description to explain its meaning. Sacred groves were important to the ancient Celts, so it is no wonder that their writing/magical system was connected to trees. Although there are none that remain, Ogham was probably also carved on wood staves, and it is possible that its messages were transmitted secretly by hand gestures.
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The Forest Lord's first appearance to Aislin in Love Lies Bleeding echoes the Green Man.