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I'm confused with the following sentences.

Which one is correct?

  1. Mary saw a poster on a singing contest.

  2. Mary saw a poster about a singing contest.
Any suggestion?

Thank you.
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1. Mary saw a poster for a singing contest.-- This is the usual
2. Mary saw a poster about a singing contest.-- This is OK
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emsr2d2We do sometimes use "on" to denote "about", but not in this context.
There's no discussion on a poster. "on" suggests a more or less elaborate treatment of a topic, which can only be done when there is some discussion of the topic.

a book on The Beatles suggests a book in which you can find a good deal of informative material on various aspects of The Beatles.

Nobody ever writes a poster "on" a topic.

CJ
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We do sometimes use "on" to denote "about", but not in this context.

I bought a really good book on The Beatles.
 CalifJim's reply was promoted to an answer.
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Exactly! That's why I said "not in this context" (ie the context used by the OP).