Ogham, the medieval Irish alphabet — and the modern Celtic tree-calendar myth

Two very different things travel under the word “Ogham” today. One is a real early-medieval Irish alphabet, carved on hundreds of surviving stones and studied by philologists for two centuries. The other is a “Celtic tree calendar” or “tree horoscope” that assigns a tree and a birth-period to each letter — and that turns out to be a twentieth-century literary invention, not an inheritance from the druids. Keeping the two apart is the whole point of this piece.

Ogham the script: what is actually attested

Ogham (Old Irish ogam) is a genuine alphabet used to write the earliest form of the Irish language. Its letters are strokes and notches cut across or on either side of a central line — on standing stones, this is usually the edge of the stone itself. The surviving monumental inscriptions, of which several hundred are known mostly from Ireland (with others in Wales, Scotland, the Isle of Man and south-west England), are generally dated on linguistic and archaeological grounds to roughly the fourth to sixth centuries CE. Most are short: typically a personal name in the genitive, often “X son of Y,” marking a grave or a boundary.

The alphabet’s twenty basic letters are grouped into four families of five, called aicmi. Each letter has a name, and the traditional order begins Beith, Luis, Fearn, Sail, Nion — which is why the system is sometimes nicknamed the Beith-luis-nin after its first letters. A fifth group of five characters, the forfeda or “supplementary letters,” was added later to handle sounds the original set did not cover.

The medieval manuscript tradition

Beyond the stones, Ogham survived as a scholarly subject in medieval Irish manuscripts. The central text is the Auraicept na nÉces (“The Scholars’ Primer”), a treatise on grammar and the alphabet preserved in several manuscripts, the material of which is generally placed in the Old Irish period (its earliest strata around the seventh century, elaborated later). Alongside it are the Bríatharogaim (“word-oghams”): sets of two-word kennings, one for each letter name, that gloss or hint at the meaning of the name. Three main lists survive — associated with the names Morann mac Moín, Mac ind Óic and Cú Chulainn — and the Auraicept reports and interprets the first of them.

This is where the tree question genuinely arises. Later medieval scholars themselves came to believe that all the letter names were tree names, and read the kennings in that light. But modern scholarship, working from the kennings themselves, concludes that at most about eight of the twenty names are reliably those of trees — beith (birch), fern (alder), sail (willow) and a handful of others — while many are not trees at all. In other words, even the medieval “tree” reading was partly a later gloss laid over an older list whose logic was not purely arboreal.

The “Celtic tree calendar”: a 1948 invention

The popular “Celtic tree calendar,” “tree zodiac” or “tree horoscope” is a separate and much younger thing. It was constructed by the poet and mythographer Robert Graves in his 1948 book The White Goddess: A Historical Grammar of Poetic Myth. Graves took the Ogham letters, treated them as a sequence of trees, and mapped them onto a calendar of thirteen lunar “months,” so that each period of the year — and, in the later popular versions, each birth date — is assigned a tree.

Celtic scholars are blunt about this. The scheme is unattested before Graves; there is no surviving evidence that the pagan Irish ran their calendar off the Ogham alphabet. To make the mapping work, Graves rearranged the traditional letter order and reinterpreted some characters as “late additions,” reducing the set so that thirteen consonants could each own one lunar month. Even Graves did not present The White Goddess as conventional history — he called it a work of poetic intuition — and accounts note that his own father and other readers warned him the scheme could not be proven. Critics writing later, such as Peter Berresford Ellis on “the fabrication of Celtic astrology,” have restated the point in detail.

So the honest summary is a clean two-layer split. The alphabet: real, early-medieval, carved in stone, studied in genuine medieval treatises. The tree calendar and birth-tree horoscope: a mid-twentieth-century literary creation by one author in 1948, however widely it now circulates.

Why the distinction matters

This is not pedantry for its own sake. The tree calendar is often sold as “ancient Celtic wisdom” and used to tell people what their “birth tree” reveals about them. Presenting a 1948 poem’s device as a druidic inheritance is a factual error, and dressing it as a predictive personality system pushes past what any source supports. You can enjoy Graves’s imaginative scheme as exactly that — a modern poet’s invention — without misdescribing its age or claiming it foretells anything.

The pattern is a familiar one on this site: a real historical object acquires a modern overlay, and the overlay gets back-dated. The same thing happened with the eightfold calendar, where medieval fire-festival names were bundled with twentieth-century additions; see The Wheel of the Year, honestly dated. And it rhymes with how a much older esoteric reputation was projected back onto the tarot, discussed in a short history of tarot.

Working with Ogham honestly

If Ogham draws you, there is a great deal to do with it that stays on solid ground. You can learn to read the actual alphabet and its four aicmi. You can look at real inscriptions — many are catalogued and photographed — and see how the “X son of Y” formula works. You can read about the Bríatharogaim and appreciate the medieval kennings as poetry and word-play. If you want to use the letters decoratively or meditatively, or to keep a “tree” association because you find it evocative, that is a personal and modern choice, which is fine so long as you name it as such rather than calling it ancient.

The most respectful stance toward the tradition is also the most accurate one: treat the script as the genuine, remarkable early-medieval achievement it is, and treat the tree calendar as the twentieth-century invention it is. Both can be interesting. Only one of them is old.