A strip of fabric is inserted in-between two panels to provide flexibility; it creates a boxlike side edge between the front and back of the pillow.
GUSSET USE IN CLOTHING:
Gussets are always added into the apex of two seams. Even a small gusset added under a sleeve can add enough ease to give relief to a too tight sleeve. The reason is the comfort of the bias set of the gusset.
Historical use (i.e. earliest garment examples found): Added to clothing to make garments fit the body more comfortably. Scraps were used in undergarments limiting the size of the gussets. Larger pieces were added to outer wear so the garment could hang and wear well. Gussets were added to the crotch seams in pants so that the wearer could sit.
Vintage use: A triangle, diamond or square shaped piece added under the arm of tight fitting bodices to enable the wearer of the garment to raise and lower her arm more easily and without distorting the shape of the garment. Depending on the size and location of the gusset, it could also be used to add some additional ease through the bust area at the same time.
Contemporary use: Often found added under the arm in sports and athletic wear to give more freedom of motion to the arm of the wearer while maintaining a slimmer profile to the over all garments. Also added to the crotch of athletic pants for the same reason.