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|Manners of articulation|
A sonority hierarchy or sonority scale is a ranking of speech sounds (or phones) by amplitude. For example, if you say the vowel [a], you will produce a much louder sound than if you say the stop [t]. Sonority hierarchies are especially important when analyzing syllable structure; rules about what segments may appear in onsets or codas together, such as SSP, are formulated in terms of the difference of their sonority values. Some languages also have assimilation rules based on sonority hierarchy, for example, the Finnish potential mood, in which a less sonorous segment changes to copy a more sonorous adjacent segment (e.g. -tne- → -nne-).
Sonority hierarchies vary somewhat in which sounds are grouped together. The one below is fairly typical:
(glides and liquids)
Sound types are the most sonorous on the left side of the scale, and become progressively less sonorous towards the right (e.g., fricatives are less sonorous than nasals).
The labels on the left refer to distinctive features, and categories of sounds can be grouped together according to whether they share a feature. For instance, as shown in the sonority hierarchy above, vowels are considered [+syllabic], whereas all consonants (including stops, affricates, fricatives, etc.) are all considered [−syllabic]. All sound categories falling under [+sonorant] are sonorants, whereas those falling under [−sonorant] are obstruents. In this way, any contiguous set of sound types may be grouped together on the basis of no more than two features (for instance, glides, liquids, and nasals are [−syllabic, +sonorant]).
In English, the sonority scale, from highest to lowest, is the following:
[a] > [e o] > [i u] > [r] > [l] > [m n] > [z v ð] > [s f θ] > [b d ɡ] > [p t k] 
More finely nuanced hierarchies often exist within classes whose members cannot be said to be distinguished by relative sonority. In North American English, for example, of the set /p t k/, /t/ is by far the most subject to weakening when before an unstressed vowel (v. the usual American pronunciation of /t/ as a flap in later, but normally no weakening of /p/ in caper or of /k/ in faker).
In Portuguese, intervocalic /n/ and /l/ are typically lost historically (e.g. Lat. LUNA > /lua/ 'moon', DONARE > /doar/ 'donate', COLORE > /kor/ 'color'), but /r/ remains (CERA > /sera/ 'wax'), whereas Romanian transformed the intervocalic non-geminate /l/ into /r/ (SOLEM > /so̯are/ 'sun') and reduced the geminate /ll/ to /l/ (OLLA > /o̯alə/ 'pot'), but kept unchanged /n/ (LUNA > /lunə/ 'moon') and /r/ (PIRA > /parə/ 'pear'). Similarly, Romance languages often show geminate /mm/ to be weaker than /nn/, and Romance geminate /rr/ is often stronger than other geminates, including /pp tt kk/. In such cases, many phonologists refer not to sonority, but to a more abstract notion of relative strength, which, while once posited as universal in its arrangement, is now known to be language-specific.
Sonority in phonotactics
Syllable structure tends to be highly influenced and motivated by the sonority scale, with the general rule that more sonorous elements are internal (i.e., close to the syllable nucleus) and less sonorant elements are external. For instance, the sequence /plant/ is permissible in many languages, while /lpatn/ is much less likely. (This is the sonority sequencing principle). This rule is applied with varying levels of strictness cross-linguistically, with many languages allowing exceptions: for example, in English, /s/ can be found external to stops even though it is more sonorous (e.g. "strong", "hats").
In many languages the presence of two non-adjacent highly-sonorous elements can be a reliable indication of how many syllables are in the word; /ata/ is most likely two syllables, and many languages would deal with the sequences like /mbe/ or /lpatn/ by pronouncing them as multiple syllables, with syllabic sonorants: [m̩.be] and [l̩.pat.n̩].
- Selkirk E (1984). "On the major class features and syllable theory". In Aronoff & Oehrle.