Kaf is the twenty-second letter of the modern Arabic order and the eleventh of the older abjad order, from which it takes the numeral value twenty. In isolation it is a tall angular stroke that turns along the line, with a small hooked mark drawn inside the angle; joined in the middle of a word it flattens into a low shape with a flag rising from it, and the internal hook is often written as a separate small stroke or left out in rapid hands.
Kaf is a moon letter, so the lam of the definite article is pronounced before it. Its sound is the voiceless velar stop, the k of English. Arabic distinguishes it from qaf, made further back at the uvula, and the two letters are separate items in the language rather than variants: swapping one for the other changes the word.
Its name descends from the Northwest Semitic kaph, the cognate of Hebrew kaf and Syriac kaph, whose sense was the palm of the hand, and it holds the numeral value twenty that Hebrew kaf holds. Persian, Urdu and the other languages written in this script built their letter for g on the kaf skeleton by adding a second stroke to it, which is why a Persian page shows a letter an Arabic reader will not recognise sitting where a kaf is expected.
Kaf is one of the fourteen letters that appear disjoined at the head of a sura, where it stands in a group of five letters opening one of them.