Examples and Observations
"Certain English nouns ending in the consonant /f/ form their plurals with /v/ instead: leaf but leaves, knife but knives. We say that such items exhibit an /f/-/v/ alternation. . . .
"A somewhat different alternation is found in related words like electric (which ends in /k/) and electricity (which has /s/ instead of /k/ in the same position).
"More subtle is the three-way alternation occurring in the English plural marker. The noun cat has plural cats, pronounced with /s/, but dog has plural dogs, pronounced with /z/ (though again the spelling fails to show this), and fox has plural foxes, with /z/ preceded by an extra vowel. This alternation is regular and predictable; the choice among the three alternants (as they are called) is determined by the nature of the preceding sound."
(R.L. Trask, Language and Linguistics: The Key Concepts, 2nd ed., ed. by Peter Stockwell. Routledge, 2007)