Reinöhl mentions that not only have the Dravidian languages borrowed from Sanskrit vocabulary, but they have also impacted Sanskrit on deeper levels of structure, "for instance in the domain of phonology where Indo-Aryan retroflexes have been attributed to Dravidian influence". Hock et al. quoting George Hart states that there was the influence of Old Tamil on Sanskrit. Hart compared Old Tamil and Classical Sanskrit to arrive at a conclusion that there was a common language from which these features both derived – "that both Tamil and Sanskrit derived their shared conventions, metres, and techniques from a common source, for it is clear that neither borrowed directly from the other."