Here is an excerpt of Petzold's Menuett in G major and I stumbled upon a mordent which I don't know how to play. The fun fact is: The excerpt is from a beginner book where there is an explanation of how to play the mordent :)
Regarding the right hand the mordent is placed on b, so I play the c and then the mordent. The mordent is explaint at the bottom of the book but beside the fact that it is awkward to play (first note of given bar is a c and the first "mordent" is also a c) it also looks wrong.
When I look the mordent up on wiki http://en.wikipedia.org/wiki/Mordent I see the upper mordent here. Both the upper and lower mordent are however played with the original note first. In my case it would be h-c-h, which flows much better.
Now the gotcha is that the menuett was written somewhere in the lifetime of Petzold :), between 1677-1733 which was in the Baroque period. The same article above says that in this time a mordent was a lower mordent instead of an upper mordent. So I would play the h as h-a-h.
I'm fine with both however as I am a novice I cannot explain why the editor of the book "helped" me by showing something that rather looks like a trill and in my opinion is a tone higher as expected.
There is a german chart of Bach's ornaments (http://de.wikipedia.org/w/index.php?title=Datei:Manieren.png&filetimestamp=20090112055252&) which confused me even more as I am unsure what the heck is what :)