Dieline / cutting template

A dieline is the engineering drawing for a printed object, laid out flat. It is a vector file where each line style and swatch name means a different machine action: cut, crease or fold, perforation, glue tab. The conventions are vendor-specific, and they conflict happily (one converter's dashed line is another's crease, another's bleed edge), so the legend that arrives with the file is the authority, not a rule you read somewhere. The printer supplies it; you drop artwork underneath it. Structural people call the flat shape a blank or a net, and standard carton geometries have catalog names (an FEFCO 0201 is the ordinary shipping box).

You need one for anything folded or shaped: a carton, a mailer, a folder with pockets, a label with a shaped edge. Working without one means guessing panel widths, and panels are not equal: the tuck panel is shorter than the face, and every fold eats a caliper's worth of material. Design on the dieline from the first sketch, not after the layout is finished.

Gotcha: the dieline is a guide, never printed ink. Keep it on its own top layer with the swatch names, line styles, and geometry exactly as supplied, stroke only, output the way the printer specifies (a nonprinting layer or an excluded separation, their call rather than yours), and delete nothing when you send the file. The other classic miss is panel orientation: artwork on a lid or a back panel often needs rotating 180 degrees so it reads right side up once the box is assembled. Fold a paper mockup before approving anything.