Beithe

	Aicme Beithe		Aicme Muine
ᚁ	Beith	ᚋ	Muin
ᚂ	Luis	ᚌ	Gort
ᚃ	Fearn	ᚍ	nGéadal
ᚄ	Sail	ᚎ	Straif
ᚅ	Nion	ᚏ	Ruis
	Aicme hÚatha		Aicme Ailme
ᚆ	Uath	ᚐ	Ailm
ᚇ	Dair	ᚑ	Onn
ᚈ	Tinne	ᚒ	Úr
ᚉ	Coll	ᚓ	Eadhadh
ᚊ	Ceirt	ᚔ	Iodhadh
	Forfeda
ᚕ	Éabhadh
ᚖ	Ór
ᚗ	Uilleann
ᚘ	Ifín	ᚚ	Peith
ᚙ	Eamhancholl

Beith is the Irish name of the first letter of the Ogham alphabet, ᚁ, meaning "birch". In Old Irish, the letter name was Beithe, which is related to Welsh bedw(en), Breton bezv(enn), and Latin betula. Its Proto-Indo-European root was *gʷet- 'resin, gum'. Its phonetic value is [b].

The Auraicept na n-Éces contains the tale of the mythological origins of Beith

On that account, moreover, b, birch, takes precedence, for it is in birch that Ogham was first written.

In the medieval kennings, the verses associated with Beith are:

Peith ᚚ, a late addition to the Forfeda, is a variant of Beith, with a phonetic value of [p], also called beithe bog "soft beithe", [p] being considered a "soft" variant of [b]. It likely replaced Ifín ᚘ, one of the "original" five Forfeda. Prior to the addition of the Forfeda to the original twenty letters, both [p] and [b] were probably symbolized by the same letter: Beith.

...
Wikipedia