Every ogham letter is identified by its family and by a count of strokes, and nothing else.
How the letter is formed
Fearn is the third letter of the first family, Aicme Beithe, and is cut as three strokes on that family's side of the line. On a standing stone the line is the arris of the stone itself and the inscription is normally read from the bottom upward.
The sound that moved
This is one of the letters where the epigraphic record and the manuscript tradition disagree about the value, and the disagreement is a fact about the Irish language rather than about the alphabet. In the language of the inscriptions the letter stands for a w sound. That sound became f in Old Irish by a regular change, and the manuscripts, written in the later language, name the letter Fearn and give it the value F.
The letter did not change. The pronunciation of the word it was named after did, and the name went with it. Anyone reading an early inscription with the manuscript values in hand will misread this letter, which is one of the reasons the two traditions are kept apart in this atlas.
Where it appears
Fearn occurs in the surviving inscriptions, which consist almost entirely of personal names in the genitive case, cut in Ireland and in Irish-settled districts of Britain between roughly the fourth and sixth centuries. The alder that later glosses attach to the letter belongs to the manuscript layer and is recorded separately.