Beith is the Irish

Irish language

Irish , also known as Irish Gaelic, is a Goidelic language of the Indo-European language family, originating in Ireland and historically spoken by the Irish people. Irish is now spoken as a first language by a minority of Irish people, as well as being a second language of a larger proportion of...

 name of the first letter of the Ogham

Ogham

Ogham is an Early Medieval alphabet used primarily to write the Old Irish language, and occasionally the Brythonic language. Ogham is sometimes called the "Celtic Tree Alphabet", based on a High Medieval Bríatharogam tradition ascribing names of trees to the individual letters.There are roughly...

 alphabet, , meaning "birch

Birch

Birch is a tree or shrub of the genus Betula , in the family Betulaceae, closely related to the beech/oak family, Fagaceae. The Betula genus contains 30–60 known taxa...

". In Old Irish, the letter name was Beithe, which is related to Welsh

Welsh language

Welsh is a member of the Brythonic branch of the Celtic languages spoken natively in Wales, by some along the Welsh border in England, and in Y Wladfa...

 bedw(en), Breton bezv(enn), and Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 betula. Its Proto-Indo-European

Proto-Indo-European language

The Proto-Indo-European language is the reconstructed common ancestor of the Indo-European languages, spoken by the Proto-Indo-Europeans...

 root was * 'resin, gum'. Its phonetic value is [b].

The Auraicept na n-Éces

Auraicept na n-Éces

Auraicept na n-Éces is claimed as a 7th century work of Irish grammarians, written by a scholar named Longarad....

contains the tale of the mythological

Celtic mythology

Celtic mythology is the mythology of Celtic polytheism, apparently the religion of the Iron Age Celts. Like other Iron Age Europeans, the early Celts maintained a polytheistic mythology and religious structure...

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

Féocos foltchaín: "Withered foot with fine hair" (Word Ogham of Morann mic Moín)

Glaisem cnis: "Greyest of skin" (Word Ogham of Mac ind Óc)

Maise malach: "Beauty of the eyebrow" (Word Ogham of Culainn)

Peith , a late addition to the Forfeda

Forfeda

The Forfeda are the "additional" letters of the Ogham alphabet, beyond the basic inventory of twenty signs. The most important of these are five forfeda which were arranged in their own aicme or class, and were clearly invented in the Old Irish period, several centuries after the peak of Ogham usage...

, 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

Ifín

Ifín is one of the forfeda, the "additional" letters of the Ogham alphabet. Its sound valueaccording to the Auraicept na n-Éces, De dúilib feda and In Lebor Ogaim, are io, ía, and ia, respectively....

, 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.