languages are spoken in the Valley of Oaxaca, and Isthmus Zapotec languages are spoken in the Isthmus of Tehuantepec. However, Valley Zapotec and Isthmus Zapotec group together (as Central Zapotec), and this ignores the Papabuco and Western Zapotec varieties. Conservative and innovative characteristics Certain characteristics serve to classify Zapotec varieties in ways that cross-cut the geographical divisions. One of these is the distinction between disyllabic roots and monosyllabic roots. Proto-Zapotec had disyllabic roots; the vowel of the second syllable could be any one of the inventory of vowels. One innovation shared by many varieties of Zapotec is the loss (or partial loss) of the vowel of the second syllable. The word for 'water' illustrates this fact. In conservative varieties, the vowel of the second syllable is retained: in Isthmus Zapotec and in Sierra de Juárez Zapotec, for example. In innovative varieties, the vowel of the second syllable was lost: in Amatlán Zapotec and Mitla Zapotec, for example. The loss of the vowel often resulted in palatalized consonants, and the loss of often resulted in labialized consonants. Compare the words for 'dog' in conservative varieties (Isthmus , Sierra de Juárez ) and innovative varieties (Amatlán and Mitla ). In this particular word Amatlán does not have a labialized consonant at the end, and the otherwise innovative variety Yatzachi keeps the final vowel: . Another characteristic that classifies Zapotec varieties is the existence or not of a contrast between alveopalatal fricatives and retroflex fricatives. Innovative varieties have introduced the contrast while conservative varieties have not. Phylogenetic classification The most influential classification of Zapotec languages is due to Thomas Smith Stark, who proposed the following overall classification of Zapotec languages. Overall family tree and Central Zapotec branch The branch of the family that contains the most languages is Central Zapotec, which includes most of the Zapotec languages of the Valley of Oaxaca and the Isthmus of Tehuantepec. The following figure shows the classification suggested by Smith Stark (2007). Northern Zapotec branch The Northern branch is shown in more detail below, again following Smith Stark (2007) Mutual intelligibility and regional classification Based on intelligibility studies, previous classifications, and the needs for literacy development, Merrill (2008) classifies the varieties as follows; several varieties (in brackets) often small and moribund, were not included in the principal list: Central Zapotec Isthmus Guevea, Isthmus Zapotec (Juchitán), Lachiguiri, Petapa, Quiavicuzas Valley Quiatoni–Mitla, Albarradas, Guelavia–Güila–?Chichicapan, San Antonio Ocotlan?–Tilquiapan–Yatzeche (Zagache)–[Zaachila], ?