Print Dialog Box - Collate Copies Option
This option is dimmed except when printing multiple copies of a double page spread as two separate pages. With this option selected, pages are printed as left-right-left-right... etc. With this option deselected, all the left-hand pages are printed, followed by all the right-hand pages. Printers that hold a page in memory (such as PostScript printers) take some time to process the image. The printer can then print multiple copies at its maximum speed. For such printers it is often faster to manually collate the pages after printing.