Phonology is the study of a sound system of language. British Council defines phonology here:
https://www.teachingenglish.org.uk/article/phonology
“Phonology, also known as phonemics, is the study of the particular sound units (phonemes) in languages. It can be compared to phonetics, which is the study of human speech in general, and includes the articulation and perception of sounds.
Example:
Phonology aims to describe the sounds (phonemes) that are distinctive in a language. This can be done by identifying minimal pairs such as ‘tin’ and ‘bin’.”
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