OghamAlphabet

Ogham is an early medieval Irish alphabet of twenty letters (plus later additions), carved as strokes along a stem line. It is attested on some 400 stones, mostly 5th-6th century, mostly personal names.

The "tree alphabet" (each letter named for a tree) rests on the medieval Auraicept na nEces and Robert Graves's "The White Goddess". The tree names are real for some letters (beith/birch, dair/oak, coll/hazel) and later inventions for others. Use it for meditation and craft; do not cite it as ancient calendar science.

First aicme: B L F S N. Second: H D T C Q. Third: M G NG Z R. Fourth (vowels): A O U E I.

Good source: Damian McManus?, "A Guide to Ogam" (1991). Bad source: almost everything on the internet.