Wings (aerodynamic type, not the feathered type)

Can anyone explain to me (or point me in the right direction) on what the difference is between a single and dual plane wing?

Found an old thread on this subject that might be of interest. It begins with single element wings then moves onto multi-elements. :)

Starts at the following post: http://www.ten-tenths.com/forum/showthread.php?postid=1155293#post1155293

Cheers, off for a read :)