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.