Myself, along with lots of other people, usually 
flip the sketch and use it as a background when 
scanning a cel. Sometimes the drawing shows 
through as a reversed image when this is done. 
This doesn't automatically mean that if you see 
the reversed image that the sketch and cel is 
stuck.  More information on sketches is at  
Yann's Guide The exact terminology of each 
type of sketch is a matter of debate. There are 
many different development levels of sketches, 
and a variety of purposes (cel development, 
scene layout, background development)

Some studios keep the sketches together with the 
cels or scene, and some separate them. Sometimes 
they can be purchased separately. animechaos 
sells sketches, for example.

Hope this helps.

- Karen