The Quran's text is universally agreed among Muslims, but its recitation was never reducible to a single fixed sound: the consonantal skeleton the earliest manuscripts preserve can be voweled and articulated in more than one way consistent with that same skeleton, and different reciting communities across the early Islamic world had transmitted their own established ways of doing so, each traced back through a chain of named transmitters to a Companion of the Prophet or to the Prophet himself. In the tenth century the Baghdad based scholar Ibn Mujahid undertook to settle how many such readings should be recognized as canonical, and in his Kitab al-Sab'a fi al-Qira'at selected seven, associated with seven reciters from different regional centers, as the readings he judged to meet the standard of reliable, continuous transmission, tawatur, that Islamic scholarship required of a reading before it could be treated as authoritative rather than merely attested.
Ibn Mujahid's choice of seven, later supplemented by three further readings scholars judged to meet the same standard, drew directly on the methodology hadith scholarship had already developed for verifying a chain of transmitters, applied here to how the Quran itself may be voiced rather than to a report about the Prophet's words. This group holds the documented transmission chains behind those readings, the evidence on which each reading's own claim to authenticity rests.