Gussets are inset patches that make more room in the under arm or crotch area of a garment, allowing a better range of motion for the arms and legs. Gussets can be oval, diamond, or square in shape. Using a square patch to illustrate: one point will meet the underarm seam of the bodice; the opposite point will meet the underarm seam of the sleeve. The remaining two points of the patch join the front and back of the armhole and are set into the sleeve seam. In the case of a tightly fitted kimono or Magyar sleeve, where the bodice and sleeve are one piece, it is necessary to cut into the fabric of the front and back bodice, so measure carefully and make a sample first.
A 60 degree diamond shape used for patchwork can help with gusset practice. This one was cut from clear acetate, the holes are for marking the end of each seam.
One way to practice setting in gussets is to make a piece of patchwork. A 60 degree diamond shape is a good size to use and only two of the diamond shapes are needed. Use a fine line permanent marker or a white chalk pencil to mark a tiny dot at the four corners where the ¼ inch seams intersect. Place two pieces together matching the dots at the narrow 60 degree point, a single pin should suffice to hold the two small pieces together. Set the work to the right side and under the presser foot. You are going to sew toward that point for about 5 stitches, then pivot the joined piece around the needle so that the work is on the left side of the presser foot. You are now sewing back over the initial stitches. Continue on to the next dot and gently back stitch. Take the piece out and cut the thread. You have sewn one side of a gusset.
Gusset patches are often used for mending damage caused by strain. Worn material is removed and a patch of new material is sewn in its place. The patch itself should lay flat.
Clothing Construction, Revised Edition. Brown, Gorham, Keever. 1927, 1934
Another inexpensive way to practice gussets is to use a worn shirt that you can cut into without worry. Thankfully Mildred Graves Ryan’s 1954 classic, Thrift With A Needle: The Complete Book of Mending is now available at Hathitrust. She also recommends “stitching one side at a time, making sure that the stitching comes exactly to the corner of the opening.”
“Worn underarm. An inset patch may be used to mend the worn area under the arm. Trim away the worn part of the sleeve to form a square hole. Each corner of the square should come at the seam line. The edges must be straight. Turn the shirt to the wrong side. Rip each seam back ½ inch so that the raw edges of the opening may be turned to the wrong side ½ inch. Baste the turned edges and press.”
Art and Picture Collection, The New York Public Library. “Sun-Burst Patchwork.” The New York Public Library Digital Collections. 1889. http://digitalcollections.nypl.org/items/510d47e1-1655-a3d9-e040-e00a18064a99