Ogham is written along a line. On a standing stone that line is the arris, the edge where two faces of the stone meet, and the letters are cut across or beside it. In a manuscript the line is drawn.
How the letter is formed
Luis is the second letter of the first family, Aicme Beithe, so it is cut as two strokes on the first family's side of the line. On a stone read upward along the left edge those strokes fall to one side of the arris; in the horizontal layout used by the manuscripts, the same family is written below the line. Which physical side counts as which depends on how the reader is standing relative to the stone, which is why the manuscripts, working on a flat page, had to fix a convention that the stones never needed.
Position is the whole alphabet
The system encodes a letter with two facts and no more: which of the four families it belongs to, and how many strokes it has, from one to five. Twenty letters follow from four families of five. There is no separate shape to memorise for any letter, which is what makes ogham unusually easy to cut and unusually easy to read once the families are known.
The sound
Luis carries the value L. It is well attested in the surviving inscriptions, which are overwhelmingly personal names in the genitive, cut in Ireland and in the Irish-settled parts of Wales, Scotland and the Isle of Man, and dated on linguistic grounds to roughly the fourth to the sixth centuries.
The tree glosses that give this letter a rowan belong to the manuscript tradition, several centuries later than the stones, and are treated separately in this atlas.