Like all modern Celtic languages, the Breton language knows oral alteration phenomenons, the consonant mutations and the softening links which makes it a very linked language.

For example,

• the word tad#Breton ("father") becomes ma zad ("my father"), da dad ("your father").
• the word penn#Breton ("head") becomes ma fenn ("my head"), da benn ("your head").

The Breton mutations have several functions:

• the linking mutations; these occur systematically after certain words, called mutators (there are about one hundred of them in Breton)
• the gender-distinctive mutations; these occur:
• in the nouns after the article (in function of its gender and its plurality) as well as in epithetic adjectives (under certain conditions)
• after the possessive pronoun of the third-person singular
• the mutations of recognition: these allow to identify homonyms correctly and are indispensable for the comprehension of a phrase.

These mutations can be divived into four categories, according to the phonetic transformations they make:

• the soft mutations
• the hard mutations
• the aspirant mutations
• the mixed mutations (containing three soft and one hard)

The mutations should not be confused with sandhi: mutations have a grammatical role and only occur at the initial consonant while sandhis are uniquely due to phonetics. Breton also knows sandhis, but those are not written.

Types of mutations

Consonant mutations of Breton are divided into several types according to the phonetic phenomenons they cause.

This classification corresponds to mutations listed by the grammars and which are regularly written. Others exist too, but are not written (see the not written mutations).

The mutations are summarised in the following table:

not mutated	soft	aspirant	hard	mixed
p	b	f
t	d	z
k	g	c'h
b	v		p	v
d	z		t	t
g	c'h		k	c'h
gw	w		kw	w
m	v			v

Soft mutations

It consists of a softening of hard consonants into soft consonants. It concerns seven consonants:

• three unvoiced plosives (which become voiced): P/B, T/D and K/G
• three voiced plosives (which become fricatives): B/V, D/Z et G/C'H (exception: Gw/W)
• one nasal: M/V

The softening mutations are by far the most frequent in Breton: these occur in the gender-distinctive mutations as well as in the big majority of the linking mutations.

Example: bihan ("small, little") becomes re vihan eo ("they are too small").

Those mutations occur after:

• about hundred linking mutators (holl, the adverb re, ...)
• the article (under conditions)
• the prepositions da, dre, a, war, dindan, eme, en ur, ...
• the interrogative pronoun pe
• the possessives da, e
• the verbal particles a, ne, na
• the numerals daou and div
• the conjunction pa
• the reflexive form en em
• the pronouns hini and re for the feminine nouns
• ...

Hard mutations

This consists of a hardening of soft consonants into hard consonants: B/P, D/T and G/K. This corresponds to a devoicening of these consonants.

Examples:

• daouarn ("hands", masculine) becomes ho taouarn ("your hands")
• breur ("brother", masculine) becomes ho preur ("your brother")
• bag ("boat", feminine) becomes ez pag ("in your boat")

These mutations occur after:

• the possessives ho, ez, da'z, az

Aspirant mutations

The aspirant mutations transform three unvoiced plosive consonants into fricatives: P/F, T/Z and K/C'H.

Examples:

• ki ("dog") becomes ma c'hi ("my dog")
• tad ("father") becomes he zad ("his father")
• paotr ("son, boy") becomes o faotr ("their son")

These mutations occur after:

• the numerals tri/teir, pevar/peder, nav
• the possessives hon (only in trégorrois), ma, em, he and o
• the day Sul for Easter (Sul Fask - "Easter Sunday")

Mixed mutations

The mixed mutations contains three soft mutations and one hard mutation: B/V, D/T, G/C'H (Gw/W) and M/V.

Example: mont ("to go") becomes emaon o vont da Vrest ("I am going to Brest" - no future tense, but "in the process of...").

Those mutations occur after:

• the verbal particles e and o ("to" + verb)
• the conjunction ma

Functions of the mutations

The consonant mutations play a role in the syntax and the grammar of Breton. They fill up several functions.

Linking mutations

These occur systematically after certain words, called mutators (there are about one hundred of them in Breton). They are always soft mutations.

Examples:

• baraerien ("bakers") becomes ar gwir varaerien ("the real bakers")
• tad ("father") becomes da dad ("your father")
• mamm ("mother") becomes da vamm ("your mother")

Gender-distinctive mutations

These occur after the article in function of the gender and the plurality of the noun as well as in the adjective after the noun (under certain conditions). These are always soft mutations.

• A noun in feminine singular mutates systematically.
• In plural, only masculine nouns of persons mutate.
• Except the rare ones on -où like tadoù.
• After a mutatable noun, the epithetic adjective mutates.
• Except for p, t and k if the noun ends on l, m, n, r or a vowel.
• All other nouns only make the mutation K/C'H (e.g. ki, "dog", becomesur c'hi) - this distinguishes Breton from Welsh.

Examples:

• paotr#Breton ("boy", masculine): ur paotr brav ("a nice boy") but ar baotred vrav ("the nice boys")
• bro#Breton ("country", feminine): ar vro vihan ("the small country") but ar broioù bihan ("the small countries")
• tad#Breton and mamm#Breton: an tad kozh ("the grandfather") and ar vamm gozh ("the grandmother")

There are exceptions, like plac'h which doesn't mutate, although feminine, but requires a mutation of the epithetic adjective: ur plac'h vrav ("a nice girl") but ur verc'h vrav.

Mutations of recognition

These allow to identify correctly homonyme words and are indispensable for the comprehension of the oral phrase.

These are generally hard, aspirant or mixed mutations.

Examples:

• e vreur ("his brother", masculine) but he breur ("her brother")
• o zi ("their house", masculine) but ho ti (" your house")

The writing of the mutations

Written mutations

In Old and Middle Breton, it was extremely rare to write the consonant mutations. Around the 17th century, the Jesuits started to learn Breton and introduced the writing of mutations.

Sometimes (generally for the proper nouns), the mutated letter is written before the not mutated letter to make reading it easier. Example: Itron vMaria ("the virgin Maria") is pronounced as /intron varia/.

Not written mutations

However, certain mutations are not written: CH/J, F/V, S/Z, hard C'H to soft C'H, ... Speakers often don't even notice these mutations...

These are mainly fricatives which mutate into other (not written) fricatives.

It actually is about relatively new mutations, what explains why they were historically not written. Moreover, they are generally not used in the vannetais dialect. Finally, they aren't gender-distinctive and are (relatively) automatic for the words they affect. Consequently, these can be called sandhi rather than mutations. They are not systematic in the sense that it doesn't affect all words starting with f, s, ...

For example, the words sellout, chom, ... are affected by these sandhis by the majority of the speakers.

More information can be found in the thesis of François Falc'hun: "Le système consonantique du breton".

The ancient nasalising mutations

Middle Breton knows nasalising mutations, like Welsh still does. However, these have become extremely rare in Modern Breton; at the point of disappearing.

The only notable exception which is staying is dor ("door") to an nor ("the door"), the only one still being written. But locally, other words have their consonant d nasalised after the article: den ("person") to an nen ("one"). This is because the nasalisation of d was one of the most frequent ones in Middle Breton.

The nasalisation of b can still be heard in "bennak(et)" ("quelque") as /m'nak(ət)/.