Verb stems are inflected through a number of different processes: the initial consonant of the verb root changes according to a morphophonemic pattern of consonant mutations to mark present vs. non-present, and active vs. passive. Verbal roots may take a formative syllable or not depending on syntactic and prosodic factors. A nasal prefix may be added to the root to express reciprocality or middle voice. Some dialects, notably the eastern ones, have a system of verb classes that take different series of prefixes. These conjugational categories have been lost in the Western dialects, although they existed in the Western areas in the colonial period as can be seen from Cárceres's grammar.