Back Seal Bags.

A bag made from a single continuous sheet of film wrapped into a tube, sealed down the back, then sealed across the top and bottom. A packet of crisps is the example everyone has handled — the seam running down the reverse is what identifies it.

It is also called a pillow pack, from the shape it takes when filled, and either a fin seal or lap seal bag depending on how the back seam is formed.

The defining characteristics are that it uses less film than any other construction and that it is normally formed on the filling line itself rather than supplied as a finished bag.

How the two edges of the film meet at the back.

A fin seal brings the edges together face to face and seals them, leaving a small standing fin on the reverse. Because inside seals to inside, only the inner layer of the laminate needs to be heat-sealable. It is more forgiving and it is what most work uses.

A lap seal overlaps one edge over the other and seals them directly. The back lies flatter and it uses marginally less film — but it requires the outer surface to bond to the inner one, which restricts which structures you can use.

Unless there is a specific reason to want the flatter back, fin seal is the sensible default.

Volume economics, mostly.

It uses the least film. At the quantities snack brands run, a fraction of a cent per bag is a large annual number.

It is made on the filling line. Vertical form-fill-seal machinery forms, fills and seals in one continuous high-speed operation, with no pre-made bags to store or handle.

The front is clean. No seam crosses the face, so photographic artwork runs uninterrupted.

The products suit it. Crisps and snacks are eaten in one sitting, are low density, and are usually flushed with nitrogen — none of which need a gusset, a zipper or a standing base.