A gusset

When you make a garment to fit a body, adjustments must be made to allow for movement. A sweater is one big tube for the trunk with two smaller ones for sleeves. This pattern creates the ease by adding an additional square at the underarm, called a gusset.

MoM was a big user of gussets. Pants bought off-the-rack would rarely fit. She would cut off the extra length on the legs and make a gusset for the crotch.

I like that word.

ORIGIN late Middle English: from Old French gousset, diminutive of gousse ‘pod, shell,’ of unknown origin.