The Sonority Sequencing Principle or Sonority Sequencing Constraint is a phonotactic principle that aims to outline the structure of a syllable in terms of sonority. The SSP states that the syllable nucleus, often a vowel, constitutes a sonority peak that is preceded and/or followed by a sequence of segments – consonants – with progressively decreasing sonority values. The sonority values of segments are determined by a sonority hierarchy, though these differ to some extent from language to language. Typically they are vowel > glide > liquid > nasal > obstruent. That is, the segment order in a syllable tends to be of the pattern Ʞ-P-F-N-L-G-V-G-L-N-F-P. The fricative–plosive and nasal–plosive rankings may be reversed. Wright notes, 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 – 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" is not. Some languages possess syllables that violate the SSP 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. Some languages allow a sonority "plateau"; that is, two adjacent tautosyllabic consonants with the same sonority level. Modern Hebrew is an example of such language. A number of unrelated languages that typically follow the SSP will violate it with /s/ + stop clusters. For example, in the English word "string" or Italian "spago" the more sonorous /s/ comes before a less sonorous sound in the onset. In native English words, no phoneme other than /s/ ever violates the SSP. 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. As a result, Western Romance languages like Spanish and French will have "espada" and "épée" respectively where a non-WesternRomance language like Italian has something akin to "spada".
