Question on Century 8x10 back?

[davidk]

I purchased a Century 10A with a 8x10 back. The plaque reads "Sliding Carriage for 10A camera. Folmer Graflex Corp"

Considering that the image gets cut off when shifted to the side, why does the back slide left and right? What is the purpose?

[Mark Sawyer]

So with the back in the horizontal position, you can make two 5x7 (or 5x8) exposures on a piece of 8x10 film.

[jnantz]

these cameras sometimes have the insert that slides infront of the ground glass.
i couldn't google the right terms to find a photograph of one
but it basically would be 5x7 in your case and it blocks off half the negative
so it doesn't get exposed. it looks like a 5x7 open ended box that protrudes
out of a plate.
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[davidk]

The inserts are missing so it didn't make any sense. Thanks for the explanation