Spreads, Signatures, and Imposition

Reader Spreads
The order in which you would read the pages of a magazine, page 1, then 2, 3, 4 and so on - is called 'reader spreads'.
As you create a document, in a page layout program, you arrange the pages in reader spreads.

Printer's Spreads
Printer's spreads refers to the order in which pages are placed for printing. When you lay out an 8 page booklet, you'd see page 1 & 8 placed together.

Signature Page:
A group of pages of a publication which are printed on both sides of ONE sheet of paper (signature), and when folded, the printed piece creates a set of pages.

Printing 8 pages on each side creates a 16 page signature.

The signature must be decided before beginning the layout to be sure the total number of pages is a multiple of the signature.
An 8 page signature can have 8, 16, 24, 32 etc. pages in the publication.

Imposition
Arrangement of pages (printer's spread) in the proper sequence and orientation on the press sheet/signature to facilitate the required folding, trimming, and binding of document.

What does "2-up printer's spread" mean?
There's two parts to this -- first the '2-up' then the 'printers spreads'.
When printing on a printing press, often because the paper sizes can be much larger than with desktop printers, or the paper may be on a continuous roll, many more pages can be printed onto a single sheet.

Rather than print each actual book or magazine page on a single sheet, many pages can be grouped together to print as many pages as possible onto the paper in one pass.

The pages have to be arranged so that when the pages are cropped and folded the pages actually make up the proper reading order and this is where the '2-up' term comes from.

The 2-up means to print two pages onto a single sheet of paper, side by side.
There are also other sizes available, such as 4, 6, 9, and 16-up.
This means you can print several pages to a larger single sheet, then crop the sheet down to the final page size.

This whole process of arranging the maximum number of pages to fit onto a single sheet on the press (to get as many printed in one go as possible) and to create the printer spreads is known as Imposition.

This can get mightily complicated, when dealing with say 16-up printing and long book documents! (I wouldn't even like to think about 32-up, 700 pages!!). Fortunately, there are impostition packages available that automatically create the printers spreads etc.

What is important to note, is that most print shops prefer to impose their pages for printing in-house. As a designer, this is not your responsibility. You will create your pages as reader spreads, 1 or 2 up and allow the printer to determine the best imposition for the paper size and the press size.