unstressed and adjacent to another vowel. Some instances of the mid vowels and , determined lexically, alternate with the diphthongs and respectively when stressed, in a process that is better described as morphophonemic rather than phonological, as it is not predictable from phonology alone. The Spanish consonant system is characterized by (1) three nasal phonemes, and one or two (depending on the dialect) lateral phoneme(s), which in syllable-final position lose their contrast and are subject to assimilation to a following consonant; (2) three voiceless stops and the affricate ; (3) three or four (depending on the dialect) voiceless fricatives; (4) a set of voiced obstruents—, , , and sometimes —which alternate between approximant and plosive allophones depending on the environment; and (5) a phonemic distinction between the "tapped" and "trilled" r-sounds (single and double in orthography). In the following table of consonant phonemes, is marked with an asterisk (*) to indicate that it is preserved only in some dialects. In most dialects it has been merged with in the merger called . Similarly, is also marked with an asterisk to indicate that most dialects do not distinguish it from (see ), although this is not a true merger but an outcome of different evolution of sibilants in Southern Spain. The phoneme is in parentheses () to indicate that it appears only in loanwords. Each of the voiced obstruent phonemes , , , and appears to the right of a pair of voiceless phonemes, to indicate that, while the voiceless phonemes maintain a phonemic contrast between plosive (or affricate) and fricative, the voiced ones alternate allophonically (i.e. without phonemic contrast) between plosive and approximant pronunciations. Prosody Spanish is classified by its rhythm as a syllable-timed language: each syllable has approximately the same duration regardless of stress. Spanish intonation varies significantly according to dialect but generally conforms to a pattern of falling tone for declarative sentences and wh-questions (who, what, why, etc.) and rising tone for yes/no questions. There are no syntactic markers to distinguish between questions and statements and thus, the recognition of declarative or interrogative depends entirely on intonation. Stress most often occurs on any of the last three syllables of a word, with some rare exceptions at the fourth-to-last or earlier syllables. Stress tends to occur as follows: in words that end with a monophthong, on the penultimate syllable when the word ends in a diphthong, on the final