Before Ibn Mujahid wrote his Kitab al-Sab'a, the Book of the Seven Readings, at Baghdad in the early tenth century, the field of accepted Quranic recitation was wider and less settled than the number seven suggests. A number of respected reciters across the major cities of the early Islamic world transmitted their own carefully preserved readings of the Quran's consonantal text, readings that agreed on the wording but differed in vowelling, in the pronunciation of particular letters, in assimilation and in occasional word choice, all treated by the tradition as equally legitimate modes of reciting one text rather than as rival versions competing to be right.
Ibn Mujahid's book did not invent these readings. What it did was select seven of them, associate each with one named eponymous reader, Nafi of Medina, Ibn Kathir of Mecca, Abu Amr of Basra, Ibn Amir of Damascus, and Asim, Hamza and al-Kisai, all three of Kufa, and present that set of seven as the canonical boundary of acceptable recitation. Readers he judged respected but did not include in his seven fell outside the boundary his book drew, a selection modern scholarship has examined for what it shows about which readers Ibn Mujahid was weighing and which he set aside.
The number he fixed did not stay fixed forever. Some four centuries later Ibn al-Jazari's Al-Nashr extended the canonical count to ten, adding three more readers to Ibn Mujahid's seven. But the seven he named remain the core of the discipline of qira'at, and the fourteen specific transmitted readings that descend from his seven eponymous readers, above all the transmission of Hafs from Asim used in most printed Qurans today, still organize themselves around the boundary his book drew at Baghdad eleven hundred years ago.