Vocalization markers indicating specific vowel sounds were introduced into the Arabic language by the end of the 9th century. The first Quranic manuscripts lacked these marks, therefore several recitations remain acceptable. The variation in readings of the text permitted by the nature of the defective vocalization led to an increase in the number of readings during the 10th century. The 10th-century Muslim scholar from Baghdad, Ibn Mujāhid, is famous for establishing seven acceptable textual readings of the Quran. He studied various readings and their trustworthiness and chose seven 8th-century readers from the cities of Mecca, Medina, Kufa, Basra and Damascus. Ibn Mujahid did not explain why he chose seven readers, rather than six or ten, but this may be related to a prophetic tradition (Muhammad's saying) reporting that the Quran had been revealed in seven "ahruf" (meaning seven letters or modes). Today, the most popular readings are those transmitted by Ḥafṣ (d.796) and Warsh (d. 812) which are according to two of Ibn Mujahid's reciters, Aasim ibn Abi al-Najud (Kufa, d. 745) and Nafi‘ al-Madani (Medina, d. 785), respectively. The influential standard Quran of Cairo (1924) uses an elaborate system of modified vowel-signs and a set of additional symbols for minute details and is based on ʻAsim's recitation, the 8th-century recitation of Kufa. This edition has become the standard for modern printings of the Quran.