I feel kind of embarassed asking this..like I 
should know.  But I've been collecting anime 
cels, genga, douga, layouts, settei, hanken etc. 
etc. all those words are familiar to me.  
However, I've recently been seeing the 
term "genzu" in genga and douga auctions lately.

What exactly is "genzu"?  What makes it 
different from the rest of the type of art you 
find associated with genga, douga, layouts, and 
etc.?