The soft mutation is by far the most frequent mutation in Breton, both in terms of the number of consonants it affects and the number of environments in which it occurs.
Environments After definite and indefinite articles The definite article and the indefinite cause the soft mutation of: • Most feminine singular nouns: :: "a mother" • Masculine plural nouns denoting people: :: "the Frenchmen" Nouns beginning with
d- and a few others do not mutate after the articles. A notable exception is ("door") → .
After proclitics The following grammatical words cause mutations to a following word: • The
prepositions , , , , , , : :: "to Cornwall" • The
interrogative pronoun "what": :: "what man?" • The
possessive pronouns "your", "his": :: "your head" :: "his father" • The verbal particles , , , , , : :: "I see people" :: "do not sing" :: "may he return quickly" • The
numerals "two (masculine)", "two (feminine)": :: ' "two girls" • The
conjunctions "if, when", "or", "while" :: "he was old when he died" :: "one person or another" • The
adverb "too": :: "too small" • The
pronouns "all", "those, ones", "that, one": :: "all the houses"
After adjectives and nouns The soft mutation occurs in: • Adjectives following feminine singular nouns: :: "beautiful chair" • Adjectives following masculine plural nouns referring to people: :: "good brothers" • Nouns following adjectives: :: "in few words" These mutations are limited. When the first word ends in a vowel or it causes the soft mutation wherever possible, but when the first word ends in
any other consonant only the consonants change in the following words. ==Spirant mutation==