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Phonotactics (from Ancient Greek phōnḗ "voice, sound" and 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 phonotactical constraints.
Phonotactic constraints are highly language specific. For example, in Japanese, consonant clusters like /st/ do not occur. Similarly, the sounds /kn/ and /ɡn/ are not permitted at the beginning of a word in Modern English but are in German and Dutch, and were permitted in Old and Middle English. In contrast, in some Slavic languages /l/ and /r/ are used as vowels and many words do not contain the english equivalent of vowels.
Syllables have the following internal segmental structure:
- Onset (optional)
- Rime (obligatory, comprises Nucleus and Coda):
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.
English phonotactics 
The English syllable (and word) twelfths /twɛlfθs/ is divided into the onset /tw/, the nucleus /ɛ/, and the coda /lfθs/, and it can thus 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 learned words such as sphragistics), phonemes in a three-consonantal onset are limited to the following scheme:
- /s/ + pulmonic + approximant:
- /s/ + /m/ + /j/
- /s/ + /t/ + /ɹ/
- /s/ + /t/ + /j/ (not in most accents of American English)
- /s/ + /p/ + /j ɹ l/
- /s/ + /k/ + /j ɹ l w/
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 English, there exist fourteen constraints on phonotactics:
- All syllables have a nucleus
- No geminates
- No onset /ŋ/ or /ʒ/
- No /h/ in the syllable coda
- No affricates in complex onsets
- The first consonant in a complex onset most be an obstruent
- The second consonant in a complex onset must not be a voiced obstruent
- If the first consonant in a complex onset is an /s/, the second must be a liquid or a glide
- Substring principle
- No glides in codas
- If there is a complex coda, the second consonant must not be /ŋ/, /ʒ/, or /ð/
- If the second consonant in a complex coda is voiced, so is the first
- Non-alveolar nasals must be homorganic with the next segment
- Two obstruents in the same coda must share voicing
Sonority hierarchy 
In general, the rules of phonotactics operate around the sonority hierarchy, stipulating that the nucleus has maximal sonority and that sonority decreases as you move away from the nucleus. 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.
This said, in some cases /s/ is "invisible" to the sonority hierarchy; as a fricative, it is more sonorant than the plosive /t/. However, combinations like [stiːɫ] (steal), which violates the sonority hierarchy, are seen and are even common in English. This same property is seen in many other languages for either /s/ or /z/ and is thus a human universal.
Notes and references 
- φωνή, τακτικός. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at Perseus Project
- Laufer 1997.
- Crystal, David (2003). The Cambridge Encyclopedia of the English Language. Cambridge University Press. p. 243. ISBN 978-0-521-53033-0.
- Haspelmath, Martin; Sims, Andrea (2010). Understanding Morphology. Hodder Education. p. 214. ISBN 0340950013.
General references 
- Bailey, Todd M. & Hahn, Ulrike. 2001. Determinants of wordlikeness: Phonotactics or lexical neighborhoods? Journal of Memory and Language 44: 568–591.
- Coleman, John S. & Pierrehumbert, Janet. 1997. Stochastic phonological grammars and acceptability. Computational Phonology 3: 49–56.
- Frisch, S.; Large, N. R.; & Pisoni, D. B. 2000. Perception of wordlikeness: Effects of segment probability and length on processing non-words. Journal of Memory and Language 42: 481–496.
- Gathercole, Susan E. & Martin, Amanda J. 1996. Interactive processes in phonological memory. In Cognitive models of memory, edited by Susan E. Gathercole. Hove, UK: Psychology Press.
- Hammond, Michael. 2004. Gradience, phonotactics, and the lexicon in English phonology. International Journal of English Studies 4: 1–24.
- Gaygen, Daniel E. 1997. Effects of probabilistic phonotactics on the segmentation of continuous speech. Doctoral dissertation, University at Buffalo, Buffalo, NY.
- Greenberg, Joseph H. & Jenkins, James J. 1964. Studies in the psychological correlates of the sound system of American English. Word 20: 157–177.
- Laufer, B. (1997). "What’s in a word that makes it hard or easy? Some intralexical factors that affect the learning of words". Vocabulary: Description, Acquisition and Pedagogy. Cambridge: Cambridge University Press. pp. 140–155. ISBN 9780521585514.
- Luce, Paul A. & Pisoni, Daniel B. 1998. Recognizing spoken words: The neighborhood activation model. Ear and Hearing 19: 1–36.
- Newman, Rochelle S.; Sawusch, James R.; & Luce, Paul A. 1996. Lexical neighborhood effects in phonetic processing. Journal of Experimental Psychology: Human Perception and Performance 23: 873–889.
- Ohala, John J. & Ohala, M. 1986. Testing hypotheses regarding the psychological manifestation of morpheme structure constraints. In Experimental phonology, edited by John J. Ohala & Jeri J. Jaeger, 239–252. Orlando, FL: Academic Press.
- Pitt, Mark A. & McQueen, James M. 1998. Is compensation for coarticulation mediated by the lexicon? Journal of Memory and Language 39: 347–370.
- Storkel, Holly L. 2001. Learning new words: Phonotactic probability in language development. Journal of Speech, Language, and Hearing Research 44: 1321–1337.
- Storkel, Holly L. 2003. Learning new words II: Phonotactic probability in verb learning. Journal of Speech, Language, and Hearing Research 46: 1312–1323.
- Vitevitch, Michael S. & Luce, Paul A. 1998. When words compete: Levels of processing in perception of spoken words. Psychological Science 9: 325–329.
- Vitevitch, Michael S. & Luce, Paul A. 1999. Probabilistic phonotactics and neighborhood activation in spoken word recognition. Journal of Memory and Language 40: 374–408.
- Vitevitch, Michael S.; Luce, Paul A.; Charles-Luce, Jan; & Kemmerer, David. 1997. Phonotactics and syllable stress: Implications for the processing of spoken nonsense words. Language and Speech 40: 47–62.
- Vitevitch, Michael S.; Luce, Paul A.; Pisoni, David B.; & Auer, Edward T. 1999. Phonotactics, neighborhood activation, and lexical access for spoken words. Brain and Language 68: 306–311.