The first Native American language Whorf studied was the Uto-Aztecan language Nahuatl which he studied first from colonial grammars and documents, and later became the subject of his first field work experience in 1930. Based on his studies of Classical Nahuatl Whorf argued that Nahuatl was an oligosynthetic language, a typological category that he invented. In Mexico working with native speakers, he studied the dialects of Milpa Alta and Tepoztlán. His grammar sketch of the Milpa Alta dialect of Nahuatl was not published during his lifetime, but it was published posthumously by Harry Hoijer and became quite influential and used as the basic description of "Modern Nahuatl" by many scholars. The description of the dialect is quite condensed and in some places difficult to understand because of Whorf's propensity of inventing his own unique terminology for grammatical concepts, but the work has generally been considered to be technically advanced. He also produced an analysis of the prosody of these dialects which he related to the history of the glottal stop and vowel length in Nahuan languages. This work was prepared for publication by Lyle Campbell and Frances Karttunen in 1993, who also considered it a valuable description of the two endangered dialects, and the only one of its kind to include detailed phonetic analysis of supra-segmental phenomena.