Page Imposition is a process used to arrange pages in the correct order for certain types of publications (and respective printing methods).

Saddle Stitched publications are arranged 2 Up (2 pages on one side of a sheet of paper) as in Diagram 1 below.

Diagram 1

Pages 2 & 7 are printed on the back of the same sheet of paper as pages 8 & 1. Pages 4 & 5 are printed on the back of the same sheet of paper as pages 6 & 3.

The pages are then folded and "nested" as shown in Diagram 2 below.

Diagram 2

After the pages are nested, they are stiched (stapled) along the spine. 

Publications that are going to be printed in Saddle Stitched form need to be laid out in multiples of 4 (2 pages on each side of a sheet of paper) to ensure proper page order when printed.

You will have to insert blank pages into your publication if the total number of pages is not divisible by 4.