After the Prophet's death, the differences in the oral transmission of Quran, variant dialects, the difference of codices, the lack of orthographical signs in the text lead to the appearance of variant readings, qira'at, of Quran. The variant readings and arguments over the recognition of them and the respected criterion resulted in the establishment of the knowledge of Quran-reading. Using different sciences as grammar (syntax), hadith, jurisprudence and exegesis of Quran, readers all over the Islamic world innovated particular readings of Quran. Finally, Ibn MujĀhid, using his position as the chief reader of Baghdad and applying his political power and authority selected the present seven readings from five important places as the canonical ones and declared others as deviant (shĀdh). Besides the scholarly feature of Ibn MujĀhid, the cultural status of Baghdad, the reputation of kfifa in this field and the special condition of the fourth century, were the effective factors which Ibn MujĀhid by his intelligence and cleverness used to canonize the seven readings of Quran.