Assimilation is the influence of one sound on the neighboring one so that they appear to have similar features. Both consonants and vowels undergo changes in connected speech, but consonants are more effected by assimilation. This term is applied only to neighboring consonants. When vowels influence consonants or vice versa, we use the term accommodation.

Assimilation can be complete (articulation of one sound changes entirely under the influence of the other so that the articulation of both becomes the same: ) and incomplete (if the articulation of one sound is influenced by the other sound only in one respect. The effected phoneme retains its typical feature (tenth: ; interdental sound influences the alveola sound , so it also becomes interdental).

Assimilation can also be analyzed from the point of view of the influence. If the preceding consonant influences on the following, assimilation is progressive. If the preceding consonant is influenced by the one that follows, it is called regressive assimilation.

Welsh English

Some of the features of Welsh English are:

- Distinctive intonational differences, including a rising intonation at the end of statements - sometimes characterised as "sing-song".

- Lengthening of all vowels is common in strong valleys accents.

- The vowel /ʌ/ in English words such as "bus" is pronounced [ə], instead of the [ɐ] used in England. Thus, in Welsh English, the vowel sounds in "bus" and "the" are identical.

- In some areas, pronouncing [ɪ] as [ɛ] e.g. "edit" and "benefit" as if spelt "edet" and "benefet".

- A strong tendency (shared with Scottish English) towards using an alveolar trill [r] (a 'rolled r') in place of an approximant [ɹ] (the r used in most accents in England).

- Yod-dropping is rare after any consonant so rude and rood, threw and through, chews and choose, for example, are usually distinct.