an accepted principle

[GoodTaste]

Do you pronounce "an" as /æn/? Or as /ən/?

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Rule: an accepted principle or instruction that states the way things are or should be done, and tells you what you are allowed or are not allowed to do.

Source: Cambridge Dictionary

[GoesStation]

I use the schwa pronunciation if I'm speaking quickly. I might use either if I were speaking more slowly.