In a suelta, a sheet of paper is the basic unit of a gathering with text printed on both sides of the sheet. Imposition is the technique of properly arranging the pages in a chase, creating the form, so that once the sheets are printed and folded correctly, they are in the proper page-number order.

IMPOSITIONS IN QUARTO. For the better understanding of these patterns, I must say that, although few are unaware of the fact, in the press the first pull that is executed is called first pull; the second, second pull. I say this because in all impositions in quarto the first page should be at the foot of the diagram in the second pull, with its signature and catchword near the corner of the diagram. However, it might happen that one must print a sheet that does not have more than six or seven pages, and since the remaining page, or pages, must come out blank, and since these blank pages fall at the head of the diagram, this situation is troublesome to make register. To avoid this trouble, the first page can be put at the head of the diagram in the first pull, with its signature and catchword near the upper corner of the diagram, keeping the arrangement of the other pages in the same order I display here, which is the same as considering, or rotating, this pattern in the opposite position, and thus the blank pages will fall at the foot, so that the register is made more reliably. But make sure that if the first forme of the white paper is imposed in this fashion, it is necessary that the reiteration is also imposed by being rotated, putting page three in the same location where the first was put. If we do not respect this arrangement, everything we did will be wasted. There are four impositions in quarto. The first is a single sheet; the second is a two-sheet gathering; the third is a one-and-a-half-sheet gathering; the fourth is only half a sheet.

This long quotation is from Alonso Víctor de Paredes’ Institution, and Origin of the Art of Printing, and General Rules for Compositors [Madrid: ca. 1680]. Edited and translated by Pablo Alvarez. Ann Arbor, The Legacy Press, 2018.

When Paredes wrote his manual in the 1680s, many collected works of plays (Partes and Escogidas) had been or were about to be printed in Spain. The vast majority of the sueltas were printed later in the 18th century. However, nothing had changed from the time that Juan Párix (Johannes Parix of Heidelberg) began printing in Segovia in 1472, using type he had brought with him from Italy. Nothing much would change until after 1833, our terminus ad quem for entering material into the database. Paredes’ Rules for Compositors was the first such manual in Europe, predating Joseph Moxon’s Mechanick Exercises: Or the Doctrine of Handy-Works, which appeared as a book in 1703, after it had been serialized. Paredes’ book was not known either inside or outside of Spain because of the highly unusual circumstances of its printing. He composed the text in the compositor’s stick and printed off trial copies. To be clear, he did not a have a manuscript from which to set type. An anachronistic analogy would be to compare it to the current practice of typing text on your laptop, making corrections as you go, and printing copies as you edit the work, until you have a satisfactory final version to print. Paredes composed in the compositor’s stick, corrected, pulled proofs, and, in the end lost it all in a tragic fire. Two copies of the work survive: one in the Providence Public Library—it had been in the personal collection of Daniel Berkeley Updike, printer, and historian of typography—and the other, in the library of the University of Valencia. Pablo Alvarez, Special Collections librarian in Hatcher Library, University of Michigan, has done a remarkable job of translating this exceedingly difficult treatise and making it available for all those interested in the history of printing.

See also: double column printing [including single and triple columns]