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I just found a Bach score, and there is this squiggly line above the A eighth note in the first measure (and a few other places). This is the score. What does it mean?
That is a trill, not a mordent. It actually starts on the upper auxiliary. The table below is from the Clavier-Büchlein vor Wilhelm Friedemann Bach, i.e., it is Bach's own. You can read more about it here.
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