an - Wiktionary
https://en.wiktionary.org/wiki/an
Jun 26, 2022 · In standard English, the article an is used before vowel sounds, while a is used before consonant sounds. Alternatively, an can be found before an unstressed syllable beginning with an h-sound, as in an historic.The h may then become silent or is at least very weakly articulated. This usage is favoured by only 6% of British speakers, and is only slightly more common in writing.
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