Computational evidence that Hindi and Urdu share a grammar but not the lexicon

Hindi and Urdu share a grammar and a basic vocabulary, but are often mutually unintelligible because they use different words in higher registers and sometimes even in quite ordinary situations. We report computational translation evidence of this unusual relationship (it differs from the usual pattern, that related languages share the advanced vocabulary and differ in the basics). We took a GF resource grammar for Urdu and adapted it mechanically for Hindi, changing essentially only the script (Urdu is written in Perso-Arabic, and Hindi in Devanagari) and the lexicon where needed. In evaluation, the Urdu grammar and its Hindi twin either both correctly translated an English sentence, or failed in exactly the same grammatical way, thus confirming computationally that Hindi andUrdu share a grammar. But the evaluation also found that the Hindi and Urdu lexicons differed in 18% of the basic words, in 31% of tourist phrases, and in 92% of school mathematics terms.