In Uto-Aztecan linguistics one of Whorf's achievements was to determine the reason the Nahuatl language has the phoneme , not found in the other languages of the family. The existence of in Nahuatl had puzzled previous linguists and caused Sapir to reconstruct a phoneme for proto-Uto-Aztecan based only on evidence from Aztecan. In a 1937 paper published in the journal American Anthropologist, Whorf argued that the phoneme resulted from some of the Nahuan or Aztecan languages having undergone a sound change from the original * to in the position before *. This sound law is known as "Whorf's law", considered valid although a more detailed understanding of the precise conditions under which it took place has since been developed.