The proper recitation of the Quran is the subject of a separate discipline named tajwid which determines in detail how the Quran should be recited, how each individual syllable is to be pronounced, the need to pay attention to the places where there should be a pause, to elisions, where the pronunciation should be long or short, where letters should be sounded together and where they should be kept separate, etc. Tajwid describes rules for what noiseless aspect of inarticulating Quranic verses?