An aicme is one of the four families of five letters that make up the ogham alphabet, each family defined by how its strokes meet the writing line.
The geometry
Aicme hUatha is cut as strokes lying to the other side of the line from the first family, and not crossing it. Its five members carry one to five such strokes: Uath, Dair, Tinne, Coll and Quert, with the values H, D, T, C and Q.
On a standing stone that means the strokes run onto the opposite face from those of Aicme Beithe. In the manuscripts, where the line is drawn horizontally, this family is written above the line and the first family below it.
Two letters worth knowing about
Two members of this family carry the sharpest linguistic evidence in the alphabet. Uath is given the value H, a sound Old Irish did not have as an independent phoneme, and the letter is correspondingly rare on the stones. Quert stands for a sound that Primitive Irish distinguished from plain C and that had merged with it by the Old Irish period, which is why the letter survives in the early inscriptions and is a dead letter in the manuscripts.
Both facts are used in dating inscriptions, since the corpus is dated on linguistic grounds far more often than archaeologically.
The mirror
This family and the first are distinguished only by which side of the line the strokes fall on, so the reading direction of an inscription has to be established before either can be read at all.