The glue lap

Any carton made from a single blank has to be joined somewhere. That seam is the manufacturer's joint, and the flap that forms it is the glue lap.

What it is

A narrow extra panel on one end of the run of walls. The blank is folded and the lap is glued to the inside face of the panel at the other end, turning a flat strip into a tube. The carton is then shipped flat and squared up later — which is the whole point.

It is called the manufacturer's joint because it is made by the converter, not by whoever packs the box. It can also be stitched or taped, which is why some styles offer a glue lap width of zero.

How wide

Wide enough to hold, narrow enough not to show. For folding cartons, somewhere around 10-15 mm is typical; for corrugated, 30-40 mm, because thicker board needs more area to bond and is less forgiving about alignment.

Too narrow and the joint peels. Too wide and the lap intrudes into the box, can foul the product, and on a printed carton may show as a ridge through the face panel.

Which panel it goes on

Convention puts it on a panel the customer does not look at — usually the back — so the seam falls at a corner rather than across a printed face. It matters more than it sounds: on a straight tuck end the front panel is often the whole reason for the design, and a seam across it defeats the purpose.

Which side it is glued to

The lap goes on the inside of the panel it joins. Glued to the outside it would be visible, would catch, and would sit proud of the face. If you are looking at a 3D preview and the seam appears on the outside, that is a fault in the preview rather than a construction anyone makes.