The Hindi–Urdu controversy arose in 19th century colonial India out of the debate over whether the Hindi or Urdu language should be chosen as a national language. Hindi and Urdu are mutually intelligible as spoken languages, to the extent that they are sometimes considered to be dialects or registers of a single spoken language referred to as Hindi-Urdu or sometimes Hindustani. The respective writing systems used to write the languages, however, are different: Hindi is written using Devanagari, whereas Urdu is written using a modified version of the Arabic script, each of which is completely illegible to readers literate only in the other.