Consonant voicing and devoicing

In phonology, voicing (or sonorization) is a sound change where a voiceless consonant becomes voiced due to the influence of its phonological environment; shift in the opposite direction is referred to as devoicing or desonorization. Most commonly, the change is a result of sound assimilation with an adjacent sound of opposite voicing, but it can also occur word-finally or in contact with a specific vowel.

For example, the English suffix -s is pronounced [s] when it follows a voiceless phoneme (cats), and [z] when it follows a voiced phoneme (dogs). This type of assimilation is called progressive, where the second consonant assimilates to the first; regressive assimilation goes in the opposite direction, as can be seen in have to [hæftə].

English no longer has a productive process of voicing stem-final fricatives when forming noun-verb pairs or plural nouns, but there are still examples of voicing from earlier in the history of English:

Synchronically, the assimilation at morpheme boundaries is still productive, such as in:

The voicing alternation found in plural formation is losing ground in the modern language, and of the alternations listed below many speakers retain only the [f-v] pattern, which is supported by the orthography. This voicing is a relic of Old English, the unvoiced consonants between voiced vowels were 'colored' with voicing. As the language became more analytic and less inflectional, final vowels or syllables stopped being pronounced. For example, modern knives is a one syllable word instead of a two syllable word, with the vowel e not being pronounced. However, the voicing alternation between [f] and [v] still occurs.

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