the Mixteca Alta, the Mixteca Baja, and the Mixteca de la Costa. However, the dialects do not actually follow the geographic areas, and the precise historical relationships between the different varieties have not been worked out. The number of varieties of Mixtec depends in part on what the criteria are for grouping them, of course; at one extreme, government agencies once recognized no dialectal diversity. Mutual intelligibility surveys and local literacy programs have led SIL International to identify more than 50 varieties which have been assigned distinct ISO codes. Four Amuzgo varieties are spoken in the Costa Chica region of the states of Guerrero and Oaxaca by about 44,000 speakers. The four varieties recognized by the Mexican government are: Northern Amuzgo (amuzgo del norte, commonly known as Guerrero or (from its major town) Xochistlahuaca Amuzgo), Southern Amuzgo (amuzgo del sur, heretofore classified as a subdialect of Northern Amuzgo); Upper Eastern Amuzgo (amuzgo alto del este, commonly known as Oaxaca Amuzgo or San Pedro Amuzgos Amuzgo); Lower Eastern Amuzgo (amuzgo bajo del este, commonly known as Ipalapa Amuzgo). These varieties are very similar, but there is a significant difference between western varieties (Northern and Southern) and eastern varieties (Upper Eastern and Lower Eastern), as revealed by recorded text testing done in the 1970s. Phonology Common phonological traits All Oto-Manguean languages have tone: some have only two level tones while others have up to five level tones. Many languages in addition have a number of contour tones. Many Oto-Manguean languages have phonemic vowel nasalization. Many Oto-Manguean languages lack labial consonants, particularly stops and those that do have labial stops normally have these as a reflex of Proto-Oto-Manguean . Tone systems The Oto-Manguean languages have a wide range of tonal systems, some with as many as 10 tone contrasts and others with only two. Some languages have a register system only distinguishing tones by the relative pitch. Others have a contour system that also distinguishes tones with gliding pitch. Most, however, are combinations of the register and contour systems. Tone as a distinguishing feature is entrenched in the structure of the Oto-Manguean languages and in no way a peripheral phenomenon as it is in some languages that are known to have acquired tone recently or which are in a process of losing it. In most Oto-Manguean languages tone serves to distinguish both between the meanings of roots and to indicate different grammatical categories.