He introduced the radical symbol (√) for the square root. It is believed that this was because it resembled a lowercase "r" (for "radix"),[3][4] though there is no direct evidence.[5] Cajori only says that a "dot is the embryo of our present symbol for the square root"[6] though it is "possible, perhaps probable" that Rudolff's later symbols are not dots but 'r's.[7]