Cels Forum

 >These layers are generally
  >       back skeleton lines always on top of the image on the bottom layer, sometimes
  >       with some extra coloring or minor addition to be added to the main image layer.  
  >       What is the purpose of that?

I'ld be interested in knowing as well.  I have several cels
like this.  Mainly my Cyber Formula cels and for my St.Tail
cels with backgrounds.

The answer to the St. Tail cels is easy.  The backgrounds are
done using water colors. There are not hard lines defining
anything.  Like a wall or a door or a table. So there is 
an extra layer of black lines that are painted on a cel layer
and placed over the water colored background to give the
background it's hard lines.  The other case of the extra
black lines ontop of a cel.  These seem to give a more 
dramatic effect to the cel.  I've looked at the cels with and without
these extra black line cels and without question they look
better with the extra lines.

I really would like to know if anyone else had another answer for this.