Gusset

In sewing, a gusset is a triangular or rhomboidal piece of fabric inserted into a seam to add breadth or reduce stress from tight-fitting clothing.

[1] Gussets were used at the shoulders, underarms, and hems of traditional shirts and chemises made of rectangular lengths of linen to shape the garments to the body.

[2][3] Gussets are used in manufacturing of modern tights and pantyhose to add breadth at the crotch seam.

As with other synthetic underwear,[4] these gussets are often made of moisture-wicking breathable fabrics such as cotton, to keep the genital area dry and ventilated.

The meaning of gusset has expanded beyond fabric, broadly to denote an added patch of joining material that provides structural support.

Late medieval shirt with gussets in the seams at shoulder, underarm, and hem . From a copy of the Tacuinum Sanitatis , 14th century.