Sketches are made of the background, just as they 
are made of the cels. The "important" areas of 
the background are the sections the characters 
directly interact with or are behind... and these 
sections are traced onto the background using 
pressure. If you hold the background up to the 
light, close to your eyes, almost parallel, 
you'll see what I mean. Note that if the 
background is not interacted with by a character, 
but is just a backdrop, you probably won't see 
any indentations.

TTYL

Karen