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Parent sheets are large pre-cut pieces of paper, typically designed to fit 4-16 pages of a project on each side or 55 poker cards. They come in many weights and are often pre-coated with gloss or matte coating. One of the major factors in the cost of a printing project is how many parent sheets will be used, which is why certain standard sizes (8.5″x11″, 6″x9″, 9″x12″) and page counts (multiples of 16) are usually the most cost-effective choices since they can maximize the parent sheets used.
Once the printing is complete, the stacks of fully-printed and dried parent sheets are taken to be folded by the folding machine. Each parent sheet zooms through the machine one at a time entering into gates that are set to the proper fold size. A final fold is applied by a heavy arm, called the knife fold, that applies pressure on the fold line. The final project rolls out of the machine as a signature. The majority of projects are folded by these machines, although extremely large projects or those with unique requirements may occasionally be folded by hand.
Finally, the stacks of signatures are taken to the collation machine, which collates the separate signatures into their proper page order. The resulting new stacks of ordered signatures are now called book blocks.
Watch the Collation machine stack signatures in page order to make book blocks.
Collation machines like the one pictured below are large enough to handle books with many hundreds of pages, but books with only two or three signatures can also be collated by hand.