noun
gusset; crotch piece
A piece of fabric inserted into a garment to add room or shape, especially in the crotch of pants or underarms. Common in sewing and tailoring contexts.
このパンツにはマチが付いているので動きやすい。
These pants have a gusset, so they're easy to move in.
袖の下にマチを入れると腕が上げやすくなる。
Inserting a gusset under the sleeve makes it easier to raise your arm.
Loanword from English 'gusset', used in similar contexts but less common than マチ in everyday sewing vocabulary.
The origin is uncertain; the word is native Japanese and the kanji 襠 is a specialized character for this meaning.