It is widely used, both formally and informally, for personal letters as well as public literature, in the literary sphere and in the popular media. It is a required subject of study in all primary and secondary schools. It is the first language of most Muhajirs (Muslim refugees who fled from different parts of India after independence of Pakistan in 1947) and Biharis (coming from East Pakistan around the time of the 1971 Bangladeshi Liberation War), who form nearly 8% of Pakistan's population, and is an acquired second language for the rest. As Pakistan's national language, Urdu has been promoted to promote national unity. It is written with a modified form of the Perso-Arabic alphabet—usually in Nastaliq script.