Well... that's a very good point Peter. The reason I think is because you included "sheet music" in the search; I don't think there is any mention of "sheet music" anywhere on IMSLP... try using "score" instead (I searched for "free score IMSLP" and the IMSLP main page was the 5th result)
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The reason I never used the phrase "sheet music" is that I thought it was not entirely correct; in conservatories they never use "sheet music", but rather "scores", because saying "sheet music" has a faint connotation of popular music.
However, I think what you're pointing out is very valid, since indeed many people use the phrase "sheet music" as a synonym for scores, and so I'll try to add a meta tag to index.php which includes "sheet music".
Sphemusator: Not really... but all the major search engines (Google, Yahoo, etc.) has found the site and indexed all the pages
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