Phonotactics (in Greek phone = voice and tactic = course) deals with restrictions in a language on the permissible combinations of phonemes. Phonotactics define permissible syllable structure, consonant clusters, and vowel sequences.

For example, in Japanese, consonant clusters like are not allowed, although they are in English. Similarly, the sounds /kn/ and /ɡn/ are not permitted at the beginning of a word in English but are in German and Dutch.

Syllables have the following internal segmental structure:

• Onset (optional)
• Rime (obligatory)
  • Nucleus (obligatory)
  • Coda (optional)

Both onset and coda may be empty (a vowel-only syllable).

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 of which representations of phoneme classes may fill the cluster. For instance, English allows at most three consonants in an onset, but phonemes in such an onset are strictly limited as follows:

/s/ + voiceless stop (/p t k/) + approximant (/l ɹ j w/)

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. There are of course exceptions to this rule, but in general it holds for the phonotactics of most languages.