Resources

Accordion or Z-fold: Two or more parallel folds with each panel folding in the opposite direction. Also called a fanfold. In England, it is called a concertina fold or over-and-back fold.
Four-panel or fly fold: Formed by folding the sheet in half, creating four panels or pages.
Folio: In book production, a sheet folded once to create four pages.
French fold: A sheet of paper folded twice, with each fold at right angles to the other. This allows a sheet to be printed on one side with the printing appearing on the "cover" and "inside" when opened. Invitations are frequently done with a French fold.
Gatefold: A foldout that doubles the page width (one parallel fold in a sheet that is usually twice as wide as the trim size). A double gatefold has two parallel folds in a sheet approximately three times as wide as the trim size of the book or publication. Plan for 1/4" total gap between gate folded panels (1/8" off each folded panel).
Letter fold: Two or more parallel folds, all oriented in the same direction, with the creases wrapping around an inner leaf.
Roll or barrel fold: Two or more parallel folds, creating a kind of "spiraling" effect. Also called an over-and-over fold. Panels cannot be of equal size. Size the outer two panels of a roll folded project to the final dimension of the piece. For most projects on text weight stock, the third panel and each succeeding one thereafter should decrease in width by 3/32" (1/8" for cover weight stock). The last panel should be 1/16" shorter than the one immediately preceding it.
Trifold: Viewing from the inside, right panel folds over center panel, then left panel folds over. The right panel should be 1/16" to 1/8" shorter than center panel; left panel and center panel are the same width.

FOLDING TIPS
Start with a folded dummy: Use this as a guide to bleeds, trims, etc. when preparing the art.
End with a folded proof: This should be carefully reviewed before printing no matter how simple the job may seem.
Design: Binding is a mechanical process, usually involving multiple steps. Keep the design within achievable tolerances.