are written with a digraph (like "ph"), and others are written with more than one letter or digraph (the velar stop is written variously as "c", "k", or "q"). In some cases, they are based on their Middle Vietnamese pronunciation; since that period, ph and kh (but not th) have evolved from aspirated stops into fricatives (like Greek phi and chi), while d and gi have collapsed and converged together (into /z/ in the north and /j/ in the south). Not all dialects of Vietnamese have the same consonant in a given word (although all dialects use the same spelling in the written language). See the language variation section for further elaboration. Syllable-final orthographic ch and nh in Vietnamese has had different analyses. One analysis has final ch, nh as being phonemes contrasting with syllable-final t, c and n, ng and identifies final ch with the syllable-initial ch . The other analysis has final ch and nh as predictable allophonic variants of the velar phonemes and that occur after the upper front vowels i and ê ; although they also occur after a, but in such cases are believed to have resulted from an earlier e which diphthongized to ai (cf. ach from aic, anh from aing). (See Vietnamese phonology: Analysis of final ch, nh for further details.) Tones Each Vietnamese syllable is pronounced with one of six inherent tones, centered on the main vowel or group of vowels. Tones differ in: length (duration) pitch contour (i.e. pitch melody) pitch height phonation Tone is indicated by diacritics written above or below the vowel (most of the tone diacritics appear above the vowel; however, the nặng tone dot diacritic goes below the vowel). The six tones in the northern varieties (including Hanoi), with their self-referential Vietnamese names, are: Other dialects of Vietnamese may have fewer tones (typically only five). In Vietnamese poetry, tones are classed into two groups: (tone pattern) Words with tones belonging to a particular tone group must occur in certain positions within the poetic verse. Vietnamese Catholics practice a distinctive style of prayer recitation called , in which each tone is assigned a specific note or sequence of notes. Grammar Vietnamese, like Chinese and many languages in Southeast Asia, is an analytic language. Vietnamese does not use morphological marking of case, gender, number or tense (and, as a result, has no finite/nonfinite distinction). Also like other languages in