Wright (2004: 51–52) notes,[3] In a Sonority Sequencing Constraint that is based on perceptual robustness, a stranded consonant (one without a flanking vowel, liquid, or glide) is dispreferred unless it has sufficiently robust internal cues to survive in the absence of formant transitions.
Segments that we expect to survive without the benefits of flanking vowels, and thus be found at syllable edges with intervening stops, are the sibilant fricatives, potentially other fricatives ... and nasals.A good example for the SSP in English is the one-syllable word trust: The first consonant in the syllable onset is t, which is a stop, the lowest on the sonority scale; next is r, a liquid which is more sonorous, then we have the vowel u /ʌ/ – the sonority peak; next, in the syllable coda, is s, a sibilant, and last is another stop, t. The SSP explains why, for example, trend is a valid English word but *rtedn (flipping the order of consonants) is not.
Some languages possess syllables that violate the SSP (Russian and dialectal Arabic, for example) while other languages strictly adhere to it, even requiring larger intervals on the sonority scale: In Italian for example, a syllable-initial stop must be followed by either a liquid, a glide or a vowel, but not by a fricative (except: [ps] borrowed words like: pseudonimo, psicologia).
Latin also was able to violate the principle in this way, however the Vulgar Latin dialects that evolved into the Western Romance languages lost this ability, causing the process of I-prosthesis to occur, whereby an /i/ was inserted at the beginning of such a word, to make the /s/ instead a coda consonant rather than an onset consonant.
[5] English string, for example, would then contain a syllable [tring] with a preceding extrasyllabic s. There are some languages that violate the SSP completely, such as Dorig.