Nun is the twenty-fifth letter of the modern Arabic order and the fourteenth of the older abjad order, from which it takes the numeral value fifty. Standing alone or at the end of a word it is a deep rounded bowl dropping below the line with a single dot above it. At the beginning and in the middle of a word the bowl vanishes and the letter becomes a short tooth carrying its dot, which is exactly what ba, ta, tha and ya become as well. Five letters share that medial shape and are distinguished only by how many dots they carry and whether the dots are above or below.
Nun is a sun letter, so the lam of the definite article assimilates to it. Its sound is the alveolar nasal, the n of English.
Its name descends from the Northwest Semitic nun, the cognate of Hebrew nun and Syriac nun, whose sense was fish, and it holds the numeral value fifty that Hebrew nun holds.
No letter carries more weight in the rules of recitation. A nun without a vowel, and the nunation that marks indefiniteness at the end of a noun, behave differently according to the letter that follows, and the four ways they can behave are among the first things a student of tajwid learns by name. Nun is also one of the fourteen letters that stand disjoined at the head of a sura, where it opens one of them alone.