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Initial letter mutations (treigladau in Welsh) is a phenomenon that occurs in Welsh and other Celtic languages where the first letter of a word might change in different circumstances. There are numerous of circumstances where mutations can occur. However, some rules are very common and very well defined and will be listed below.

Overview of mutations in Welsh

Not all letters mutate in Welsh, and most letters don't have all mutations. In the table below you'll see all the possible mutations in Welsh. Initial refers to the original letter of the word i.e the one that you'll find in a dictionary. If a cell does not contain a value it is not a mutation that exist.

Initial Soft Nasal Aspirate
P B Mh Ph
T D Nh Th
C G Ngh Ch
D Dd N
G _* Ng
Ll L
Rh R
  • For soft mutation for the letter G the initial letter is simply dropped.

Soft mutation

Soft mutation is the far most common mutation and occurs in many circumstances.


  • After many prepositions, such as i(to), o(from), ar(on), and so on.
  • Feminine nouns after a definite article (y, yr, or 'r)
  • Adjectives after the particle yn (yn / 'n)


Rydw i'n mynd i Gaerdydd
Caerdydd -> Gaerdydd
I am going to Cardiff

Nasal mutation

Nasal mutation is also fairly common. The number of places it can occur is a lot less than soft mutation and the rules are very well defined.


  • After the singular first possessive pronoun fy (mine)
  • After the preposition yn (in)


Mae fy nhiwtor yn darllen
Tiwtor -> Nhiwtor
My tutor is reading
Mae hi'n byw yn Ngheredigion
Ceredigion -> Ngheredigion
She lives in Ceredigion

Aspirate mutation

Mixed mutation

Mixed mutation is a mixture of aspirate mutation and soft mutation. The premise of mixed mutation is that a word should use the aspirate mutation where possible (for words starting with p, t, and c) and otherwise it should use soft mutation.[1][2] This mutation is more common in the literary language.



  1. King G (2015), Modern Welsh: a comprehensive grammar. Routledge
  2. Lewis, D. G. (2003). Y Treigladur: A check-list of Welsh Mutations (Argraffiad Newydd). Gomer Press.