Gusset
A fold in the side of a sack or tube that lets it open square rather than flat.
Also called: gusseted, side fold
Quoted as a third number in a size: 700 × 300 × 1100 is 700 wide with a 300 mm gusset. Above about 1200 mm lay-flat width, liners are made gusseted because a plain tube that wide is unusable.