What is the difference between a sarod and a rabab?

The Afghan rabab is the immediate precursor of the sarod, carried into North India by Pashtun musicians. The rabab has a fretted wooden neck; in Amjad Ali Khan's account, his forefathers replaced it with a smooth steel fingerboard and changed gut strings for steel. That fingerboard lets a sarod player slide across many notes, which the fretted rabab cannot do.