"As T. Burrow puts it in his book The Sanskrit Language (Delhi ed. 2001: Motilal Banarsidass, p.4): The relations between the ancient Iranian and the language of the Vedas is so close that it is not possible satisfactorily to study one without the other. Grammatically the differences are very small; the chief differentiation in the earliest period lies in certain characteristic and well-defined phonetic changes which have affected Iranian on the one hand and Indo-Iranian on the other. It is quite possible to find verses in the oldest portion of the Avesta, which simply by phonetic substitutions according to established laws can be turned into intelligible Sanskrit. The greater part of the vocabulary is held in common and a large list could be provided of the words shared between the two which are absent from the rest of the Indo-European."