Azu opens the Glagolitic order, and it opens it where Greek opens. The alphabet Constantine built for the Slavs keeps the sequence of the Greek alphabet and pushes the signs it needed for Slavonic sounds into the gaps between, so a reader who knows the Greek order can follow the Glagolitic one for most of its length. The vowel azu writes is a plain low vowel, long or short by position and never marked as either, and it is among the commonest letters on a page of Old Church Slavonic, because a great many of the grammatical endings of that language finish in it.
As a numeral azu is the figure one, and the way a Glagolitic scribe wrote a numeral is worth knowing before reading any manuscript. The letter is lifted out of the words around it, commonly by a point set on each side of it and by a horizontal stroke drawn over the top, so that the eye is told at once that the sign is to be counted and not sounded. Chapter divisions, psalm numbers, the numbering of the gatherings that make up a bound codex and the figures in a colophon all appear in that form.
The letter survives in two hands that do not resemble each other closely. The rounded forms of the Bulgarian and Macedonian manuscripts and the angular forms the Croatian tradition later developed are the same alphabet drawn on different principles, one built from curves and closed loops and the other from squares, verticals and hard corners. A reader trained on one of them has to learn the other more or less from the beginning.