Reducing syllables generally consist of a stop or nasal and the vowels "ï" or "u" ("pï", "pu", "tï", "tu"..., "mï", "mu"...); "rï" and "ru" syllables can also reduce, but with some irregularities; "wï" syllables only reduce stem-initially (and apparently never have a coda grade). Historically, syllable reduction results from the weakening and loss of the high vowels "ï" and "u", leading to the formation of consonant clusters, in which the first element typically 'debuccalizes' to a glottal element ("h" or "") and later disappears, causing (when possible) the compensatory lengthening of the preceding vowel (cf. Gildea 1995). Comparative evidence suggests that many, perhaps all, morpheme-internal clusters in the Cariban family were formed as a result of this process. CV.CV.CV..." > "...CVC.CV..." > "...CVh.CV..." or "...CV.CV..." > "...CVV.CV..." In Tiriyó, as in most Cariban languages, there is a class of stems which has two forms in different morphosyntactic environments: a form which is "e"-initial (the "e"- or "front grade") and a form which is "ë"-initial (the "ë"- or "back grade"). With nouns, for instance, the back grade occurs with the inclusive (1+2) prefix "k-", the third-person coreferential ('reflexive') prefix "t-", and with the non-possessed form (prefixless); all other person-marked forms have the front grade. Tiriyó morphology is in most respects typical of the Cariban family. It is neither highly polysynthetic nor highly isolating. Tiriyó exhibits many forms of nominalization that distinguish between potential and actual Agent, Subject, and Object as well as Circumstance and Event nominalizers. It marks for possession, including past possession. Verbs also have derivational morphology. They mark for past, present, and future tense, as well as for certainty, doubt, and non-factual, hypothetical, incredulative, and admonative statements. Imperatives may also be conjugated as a hortative. Tiriyó has a wide variety of adverbial forms, and a variety of postpositions including directional, locative, perlative, relational, and experiencer.