Baghdad has always played a significant role in the broader Arab cultural sphere, contributing several significant writers, musicians and visual artists. Famous Arab poets and singers such as Nizar Qabbani, Umm Kulthum, Fairuz, Salah Al-Hamdani, Ilham al-Madfai and others have performed for the city. The dialect of Arabic spoken in Baghdad today differs from that of other large urban centers in Iraq, having features more characteristic of nomadic Arabic dialects (Versteegh, ''The Arabic Language''). It is possible that this was caused by the repopulating of the city with rural residents after the multiple sackings of the late Middle Ages. For poetry written about Baghdad, see Reuven Snir (ed.), ''Baghdad: The City in Verse'' (Harvard, 2013). Baghdad joined the UNESCO Creative Cities Network as a City of Literature in December 2015.