Phonotactics (from Ancient Greek phōnḗ "voice, sound" and tacticós, also spelled/known as taktikós "having to do with arranging") is a branch of phonology that deals with restrictions in a language on the permissible combinations of phonemes. Phonotactics defines permissible syllable structure, consonant clusters and vowel sequences by means of phonotactic constraints.
Phonotactic constraints are highly language-specific. For example, in Japanese, consonant clusters like /st/ do not occur. Similarly, the clusters /kn/ and /ɡn/ are not permitted at the beginning of a word in Modern English but are in German and Dutch (in which the latter appears as /ɣn/) and were permitted in Old and Middle English. In contrast, in some Slavic languages /l/ and /r/ are used alongside vowels as syllable nuclei.
Both onset and coda may be empty, forming a vowel-only syllable, or alternatively, the nucleus can be occupied by a syllabic consonant. Phonotactics is known to affect second language vocabulary acquisition.
The English syllable (and word) twelfths /twɛlfθs/ is divided into the onset /tw/, the nucleus /ɛ/ and the coda /lfθs/; thus, it can be described as CCVCCCC (C = consonant, V = vowel). On this basis it is possible to form rules for which representations of phoneme classes may fill the cluster. For instance, English allows at most three consonants in an onset, but among native words under standard accents (and excluding a few obscure loanwords such as sphragistics), phonemes in a three-consonantal onset are limited to the following scheme:
This constraint can be observed in the pronunciation of the word blue: originally, the vowel of blue was identical to the vowel of cue, approximately [iw]. In most dialects of English, [iw] shifted to [juː]. Theoretically, this would produce *[bljuː]. The cluster [blj], however, infringes the constraint for three-consonantal onsets in English. Therefore, the pronunciation has been reduced to [bluː] by elision of the [j] in what is known as yod-dropping.
Segments of a syllable are universally distributed following the Sonority Sequencing Principle (SSP), which states that, in any syllable, the nucleus has maximal sonority and that sonority decreases as you move away from the nucleus. Sonority is a measure of the amplitude of a speech sound. The particular ranking of each speech sound by sonority, called the sonority hierarchy, is language-specific, but, in its broad lines, hardly varies from a language to another, which means all languages form their syllables in approximately the same way with regards to sonority.
To illustrate the SSP, the voiceless alveolar fricative [s] is lower on the sonority hierarchy than the alveolar lateral approximant [l], so the combination /sl/ is permitted in onsets and /ls/ is permitted in codas, but /ls/ is not allowed in onsets and /sl/ is not allowed in codas. Hence slips /slɪps/ and pulse /pʌls/ are possible English words while *lsips and *pusl are not.
The SSP expresses a very strong cross-linguistic tendency, however, it does not account for the patterns of all complex syllable margins.[further explanation needed] It may be violated in two ways: the first occurs when two segments in a margin have the same sonority, which is known as a sonority plateau. Such margins are found in a few languages, including English, as in the words sphinx and fact (though note that phsinx and fatc both violate English phonotactics).
The second instance of violation of the SSP is when a peripheral segment of a margin has a higher sonority than a segment closer to the nucleus. These margins are known as reversals and occur in some languages including English (steal [stiːɫ], bets /bɛts/) or French (dextre /dɛkstʁ/ but originally /dɛkstʁə/, strict /stʁikt/).