Each ogham letter is a family and a stroke count, and Quert closes the second family with the maximum count.
How the letter is formed
Quert is the fifth letter of the second family, Aicme hUatha, cut as five strokes on that family's side of the line.
The letter that outlived its sound
Quert stands for a sound that Primitive Irish distinguished from plain C and that Old Irish did not. The distinction is preserved in the inscriptions, most visibly in the word for son, which appears in the early inscriptions with this letter and in the later language without it. The word runs through the surviving corpus because the inscriptions are overwhelmingly of the form name, son of, name.
By the time the manuscript tradition came to explain the alphabet, the sound was gone. Quert had become a letter with a name, a position and no living value, and the manuscripts handle it by supplying a tree gloss and moving on.
Why this matters for reading the corpus
The presence or absence of this letter is one of the criteria used to date an inscription on linguistic grounds, and the corpus is dated that way rather than archaeologically in most cases. The stones themselves rarely come with a datable context, so the language on them does the work.
The apple attached to Quert comes from the manuscript glosses, several centuries after the stones, and is recorded separately in this atlas.