A and an function as the indefinite forms of the grammatical
article in the
English language and can also represent the number one. An is the older form (related to one, cognate to German ein etc), now used before words starting with a
vowel sound (or a
syllabic consonant), regardless of whether the word begins with a vowel
letter. Examples: a light-water reactor; an LWR; a sanitary sewer overflow; an SSO; a HEPA filter (because HEPA is pronounced as a word rather than as letters); a hypothesis; an hour; a unicorn.
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