The phonological rules of a language can influence its morphological rules.
Phonotactics - the rules that govern phoneme sequences within a language.
Phonological rules not only cover phonotactics, but how to resolve potential contradictions within phonotactics.
- These rules also include where to place emphasis via accents.
- It is possible to know one of the phonotactics and the repairs for contradictions in the phonotactics, but not the other.
Like morphological rules, phonological rules have rule orderings—the enforcement of some rules is prioritized over others in case of contradictions between rules. This is called optimality theory.
- The hope of optimality theory is that there are universal constraints across languages (with languages having their own rule orderings for these constraints.)
- By specifying broad enough constraints, we can capture general rules that can be made about the language, and allow for cross-language comparison.
Phonological rules talk about sound classes

Common Rules

Devoicing - some sounds are devoiced under certain conditions, even when they normally would not be.
- Many languages devoice final obstruents.
Assimilation Rules - one sound becomes more like another sounds.
- Vowel Harmonies - vowels are articulated differently depending on the other phonemes in the word.
- Sibilant Harmonies - sibilants are articulated differently depending on other phonemes in the word.
Feature Spreading - some rules (like vowel harmony) allow a feature of a sound (i.e., roundedness) to appear in multiple places.
- Another way to say this, is that some rules may specify that for certain words, all vowels should share a feature (i.e., all vowels are back).
Many languages put their stresses as far left or as far right as possible (subject to other rules), for words that have only one stress.
Sonority Sequencing Principle - in general, for most languages, the initial sound of the syllable must rise in sonority (loudness), and the ending sound of the syllable must fall in sonority
- This is why vowels tend to be in the middle of syllables.

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