Oggham

The age-old written language of the Oggs, referenced in Lords and Ladies. The Oggs have been around in Lancre for a very long time.

On Roundworld

Ogham runes[1], or the tree-alphabet, that were beginning to emerge as a written Celtic script at the time the Celtic world was largely over-run by the Romans - they persisted for a long time in Ireland, (influenced by Nordic runes) although Britain and Gaul adopted the Latin alphabet.

Revived in our day as a part of modern Paganism and witchcraft, (of the Magrat Garlick folklorique sort)