Before printing type and blocks are placed in a metal frame, or chase. Areas that do not print are filled out with spacing material and everything is locked in place with expanding wedges called quoins. All of it together is known as a forme.
The postion of the type in the forme determines where it will appear on the paper when printed, and I take care to get it right. When I'm printing a book the pages have to be positioned so that they appear in the right order after folding. A folded dummy is an essential aid to getting that right.