Preaspiration is the puff of air that occurs before the release of a voiceless consonant. If the puff of air occurs after the consonant, it is called postaspiration. Both types of aspiration occur in Gaelic. Phonetic aspiration should not be confused with the initial consonant mutation called lenition, although the term is sometimes use to refer to the phenomenon.
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Crystal, David (2008). A Dictionary of Linguistics and Phonetics Sixth Edition. Blackwell Publishing Ltd.