Possible Mutations
Radical Soft Nasal Aspirate
p b mh ph
t d nh th
c g ngh ch
b f m  
d dd n  
g / ng  
ll l    
rh r    
m f    

Welsh, as with all other Celtic languages, often sees changes made to the beginning of words depending on the word that precedes it, or the role it plays in the sentence. These changes are known as "mutations", of which Welsh has three distinct types. Common situations in which a mutation may occur are when a word follows a preposition, possessive, or number.

The three classes of mutation are soft, nasal, and aspirate. Soft mutations are by far the most common, whereas aspirate mutations are becoming applied less and less in the spoken language. All mutations occur to words commencing with one of nine different consonants (p, t, c, b, d, g, m, ll, rh), replacing them with different consonants according to mutation type. There are two important things to note at this point:

  • ll and rh are treated as separate letters in their own right and hence have their own section in the dictionary
  • A good knowledge of the possible changes is essential when searching for a word in the dictionary: botel, mhotel, and photel – for example – are all mutated forms of the "base" word potel (meaning "bottle"), which will only be listed in this base (or radical) form in the dictionary. Similarly, knowing that the soft mutation of an initial g causes it to disappear will help to trace an instance of orsaf back to its radical form gorsaf ("station").

Soft mutation


Soft mutation occurs in many situations in the language. Here is an outline of some of the more common situations:

1. Feminine nouns: Welsh has two genders of noun but the importance of committing these genders to memory is slightly less obvious when teaching oneself than it is in other languages such as French and Spanish. While in French gender dictates a series of changes - which definite article is used (le/la), how adjectives agree (petit/petite)... - in Welsh the gender of the noun has a more subtle, but important role in constructing grammatically correct sentences.

Using the definite article y (yr before a vowel) causes a feminine noun to undergo a soft mutation (except if it begins with ll- or rh-).


  • bachgen - boy
  • merch - girl

after adding the definite article we have:

  • y bachgen - the boy


  • y ferch - the girl (As can be seen from the table above, soft mutation means m becomes f )

The same occurs when the number un - one precedes the noun (except if it begins with ll- or rh-). Hence:

  • un bachgen - one boy
  • un ferch - one girl.

Note 'a boy' would be put into Welsh as simply bachgen, there is no equivalent for the indefinite article 'a' in Welsh.

2. Adjectives following feminine nouns Continuing to use the examples above we will see how any adjective following a feminine noun will also undergo soft mutation.

  • y bachgen - the boy
  • y ferch - the girl

Adding the adjective bach (Eng. little) gives us:

  • y bachgen bach - the little boy


  • y ferch fach - the little girl ( b also mutates to f )

The difference is clear in the following two phrases:

  • Prynhawn da - Good afternoon
  • Noswaith dda - Good evening

Noswaith (evening) mutates the adjective da (good)→dda because it is feminine, while prynhawn (afternoon) - a masculine noun - does not.

3. After the majority of prepositions. Most prepositions, particularly monosyllabic ones, require a soft mutation.


  • Croeso i Gymru - Welcome to Wales (CG)
  • O Fangor i Landudno - From Bangor to Llandudno (BF and LlL)

List of prepositions triggering a soft mutation -

am : ar : at : dan  : dros : drwy : gan : heb : hyd : i : o : wrth

Welsh English base form of mutated word
am flynyddoedd for years blynyddoedd
ar daith on a journey taith
at gost o £15 at a cost of £15 cost
dan reolaeth under control rheolaeth
dros ddwy awr over two hours dwy
drwy ddull electronig by electronic means dull
gan Lywodraeth Cymru by the Welsh Government llywodraeth
heb rybydd without warning rhybydd
hyd gopa'r bryn to the top of the hill copa
i blant for children plant
o orsaf yr heddlu from the police station gorsaf
wrth greu swyddi by creating jobs creu

4. After certain possessives The pronouns for the possessives 'your' (singular, familar) and 'his' also require soft mutation. Other possessives require nasal or aspirate mutations.

dy...(di) - your (familiar)

ei...(e/o) - his


tafarn - pub


dy dafarn (di) - your pub

llyfr - book


ei lyfr (e/o) - his book

The pronoun ei also means 'her', as well as his, but in this case it requires an aspirate mutation, as is seen in the section below.

Nasal mutation


Nasal mutation is the third column in our table.

1. After the preposition yn (in).

  • This should not be confused with yn when it operates as a particle in constructing a verb. When used as a particle it does not require mutation.

Yn... - In...


  • Porthmadog - Porthmadog
  • Tresaith - Tresaith
  • Caerdydd - Cardiff
  • Bangor - Bangor
  • Dolgellau - Dolgellau
  • Glanllyn - Glanllyn

If we add the preposition:

  • Ym Mhorthmadog - In Porthmadog
  • Yn Nhresaeth - In Tresaeth
  • Yng Nghaerdydd - In Cardiff
  • Ym Mangor - In Bangor
  • Yn Nolgellau - In Dolgellau
  • Yng Nglanllyn - In Glanllyn

Notice that yn also changes before certain letters:

  • Ym - Before m-
  • Yng - Before ng-

2. After fy. The word fy, meaning "my", causes nasal mutation in the following words:

  • dillad - clothes
  • fy nillad - my clothes
  • brawd - brother
  • fy mrawd - my brother
  • tadau - fathers
  • fy nhadau - my fathers

In the spoken language, the word fy is sometimes dropped entirely:

  • nillad - my clothes
  • mrawd - my brother
  • nhadau - my fathers

Aspirate mutation


This type of mutation is shown in the fourth column of the table.

1. After a certain possessive The pronoun ei (her) causes an aspirate mutation.

ei...(hi)' - her


  • Carreg - Stone
  • Tad - Father
  • Papur - paper

After adding ei, we have:

  • Ei charreg (hi) - Her stone
  • Ei thad (hi) - Her father
  • Ei phapur (hi) - Her paper

2. After "a" a is the Welsh word for and (ac is only used before vowels, hence it cannot cause an aspirate mutation).

...a... - ...and...


  • Trosedd - Crime
  • Cosb - Punishment
  • Bara - Bread
  • Caws - Cheese

If we link two of them together with a...

  • Trosedd a chosb - Crime and Punishment
  • Bara a chaws - Bread and cheese