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I found the following sentence in my English dictionary: An attachment is an supplementary part that may be easily connected or removed.
I don't understand why "an" is used before "supplementary" in this sentence. As far as I know, "an" is used before words beginning with a vowel. Is it because "supplementary" functions as an adjective here? Could someone please explain to me?

Thanks in advance for your help!
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Welcome to the forums, daboal!

You can consider this a typographical error.
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Students: Are you brave enough to let our tutors analyse your pronunciation?
Thanks for your warm welcome, Philip, and for clearing it up.
it is a printing error. of course it should be a and not an