Whorf's law is a sound law in Uto-Aztecan linguistics proposed by the linguist Benjamin Lee Whorf. It explains the origin in the Nahuan languages of the phoneme which is not found in any of the other languages of the Uto-Aztecan 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 phoneme was a result of some of the Nahuan or Aztecan languages having undergone a sound change changing the original */t/ to in the position before */a/. This sound law has come to be known as "Whorf's law", and is still considered valid although a more detailed understanding of the precise conditions under which it took place has been developed. The situation had been obscured by the fact that oftentimes the */a/ had then subsequently been lost or changed to another vowel, making it difficult to realize what had conditioned the change. Because some Nahuan languages have /t/ where others have , Whorf thought that the law had been limited to certain dialects and that the dialects that had /t/ were more conservative. In 1978, Lyle Campbell and Ronald Langacker showed that in fact Whorf's law had affected all of the Nahuan languages and that some dialects had subsequently changed to /l/ or back to /t/, yet it remains evident that the language went through a /tɬ/ stage. In 1996, Alexis Manaster Ramer showed that the sound change had in fact also happened before the Proto-Uto-Aztecan high central vowel *//, and not just before */a/. Today the best known Nahuan language is Nahuatl.