Gusset

In sewing, a gusset is a triangular or rhomboid piece of fabric inserted into a seam to add breadth or reduce stress from tight-fitting clothing and for added strength or expansion.

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. Gussets are used in manufacturing of modern tights or underwear to add breadth at the crotch seam.

Gusset: Silk floral briefs by Stella McCartney;

Gusset: Fair Isle sweater tights by Marc by Marc Jacobs;