I have had occasion recently to apply gussets in two garments that were ready-made but didn't fit well. My cousin Sharon's granddaughter had a Halloween costume with a jacket, and the jacket was too tight in the armholes and sleeves. She asked if I could figure out a way to make the jacket more comfortable so she could avoid returning it to the catalog store where she'd ordered it.
I immediately thought of a gusset insert, which would enlarge the armscye without distorting the fit in other places in the jacket.
The term gusset in sewing is used to define a piece of fabric, sometimes diamond shaped, sometimes a triangle, that is inserted in a sewn to provide either expansion or reinforcement.
It also means the triangle that is sewn into the bottom of a bag or pillow to square the corners. I knew you did that, I just didn't realize it was a gusset!
The Miriam Webster dictionary says the word gusset comes from a Middle English term for a piece of armour covering the joints in a suite of armor, from the Anglo-French word gouseete, first used around the year 1570.
When we actually got together and I saw the jacket on Evie, I realized that the arm was too tight all the way down to the wrist. So technically I suppose the piece I inserted doesn't really conform to the true definition of gusset, but regardless, I am calling it a gusset, like the ones shown above
Of course I didn't take any pictures of the costume transformation, so I have tried to do an illustration of what I actually did. Here goes: