Like all modern Celtic languages, Breton is characterised by initial consonant mutations, which are changes to the initial sound of a word caused by certain syntactic or morphological environments. In addition, Breton, like French, has a number of purely phonological sandhi features caused when certain sounds come into contact with others. The mutations are divided into four main groups, according to the changes they cause: soft mutation (Breton kemmadurioù dre vlotaat), hard mutation (kemmadurioù dre galetaat), spirant mutation (kemmadurioù c'hwezhadenniñ) and mixed mutation (kemmadurioù mesket). There are also a number of defective (or incomplete) mutations which affect only certain words or certain letters.

Summary of sound changes

The main mutations cause the following changes:

Functions of mutations

The role which initial mutations play in Breton grammar can be divided into three categories (which are not mutually exclusive):

Soft mutation

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 al/an/ar and the indefinite ul/un/ur cause the soft mutation of: Nouns beginning with d- and a few others do not mutate after the articles. A notable exception is dor ("door") → an nor.

After proclitics

The following grammatical words cause mutations to a following word:

After adjectives and nouns

The soft mutation occurs in: These mutations are limited. When the first word ends in a vowel or -l, -r, -m, -n it causes the soft mutation wherever possible, but when the first word ends in any other consonant only the consonants g-, gw-, m-, b- change in the following words.

Spirant mutation

Environments

The mutation occurs following: In the spoken language the spirant mutation is usually replaced with the soft mutation after numerals.

Defective mutations

Hard mutation

Environments

The mutation is caused by:

Mixed mutation

Environments

The mixed mutation occurs after:

Mutations and external sandhi

All of the consonant mutations described above began as simple phonological processes in the Common Brittonic language from which Breton arose and became standardised as grammatical processes as the language developed. Similar phonological processes continued to affect Breton and cause changes to word-initial sounds, but they are usually applied based on the phonology of the preceding word and not on its function. Because of this, they cannot be described as true initial mutations and are more properly aspects of external sandhi.

Nasalisation

The true nasal mutation which occurs in Welsh never occurred in Breton and Cornish, where it was replaced by the Spirant Mutation (compare Welsh fy nghi "my dog" with Breton ma c'hi). But there was assimilation of the voiced plosives, particularly b, d to a preceding nasal and this was often written in Middle Breton. Today it is only written with an nor "the door" but can still be heard dialectally in other words, e.g. an den "one" (lit. "the person") and bennak(et) "some".

Spirantisation

Today, a number of nouns beginning with k change to c'h following the articles ar "the" and ur "a": Although this is the same process seen in the spirant mutation (e.g. following hor "our"), it is really an external sandhi which has become fixed in writing.

"Interchangeable" consonants

Breton has a series of 'interchangeable' consonants, composed of plosives and fricatives. When these sounds occur word-finally, they may be pronounced voiceless or voiced depending on the word that follows: The table below shows the 'interchangeable' consonants: These changes are never written but occur regularly, regardless of how the final consonant is spelled:

Exceptions

More information on this phenomenon can be found in the thesis of François Falc'hun: Le système consonantique du Breton.

Orthography of 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, the mutated letter is written before the radical letter in the style of the Gaelic languages, to make recognition easier. This is largely confined to proper nouns (e.g. Itron vMaria "the virgin Maria" is pronounced ). Some processes which are properly part of external sandhi have become crystallised in the written language, whilst others have not.