Ayn is the eighteenth letter of the modern Arabic order and the sixteenth of the older abjad order, from which it takes the numeral value seventy. It is a moon letter, and it joins on both sides.
No letter of the script changes more with position. Standing alone it is a hook above a deep bowl that curls back below the line. At the end of a word joined to what precedes it, the bowl remains but the hook flattens. In the middle of a word it becomes a small closed triangle on the line, and at the beginning a shallow open hook. A reader who has learned the isolated form alone will not recognise the other three, which is why teaching the Arabic alphabet from a chart of isolated letters gets a learner further with ba than with ayn.
Its sound is a voiced pharyngeal fricative, produced by constricting the throat well below the point at which any European language forms a consonant. Together with its voiceless partner, the pharyngeal ha, and with the glottal stop and the plain glottal h, it belongs to the group the grammatical tradition calls the letters of the throat. The tradition further holds that the hamza sign, which writes the glottal stop, was drawn from the head of ayn, so the two are linked in the shape of the writing as well as in the place of articulation.
Its name descends from the Northwest Semitic ayin, whose sense was eye, and it is the cognate of Hebrew ayin and Syriac e. It is one of the fourteen letters that appear disjoined at the head of a sura.