Gusset. It's one of those funny sounding sewing terms akin to godet, gore, armscye or miter that, for some of us, requires a deep search of our memory bank in order to remember its definition. Gussets are inserts of fabric, usually of rhomboidal shapes, that are set into seams of our clothing to allow our limbs more range of motion. Common placement for a gusset includes, the armpit of a dolman sleeve blouse, certain bags and purses and in the crotch of pants, which is what I'll display in this post.
You may have noticed that not all pants have crotch gussets and it's usually not necessary unless you plan on moving around in ways other than just walking and sitting like, for instance, cartwheeling to work or leap frogging with a friend to get coffee. Another reason one may find themselves desiring a gusset is that they are expanding. This is the case of my boyfriend who has been strength training for the past several months and has recently discovered that his cozy flannel, winter, lounging pants are now way too snug around his upper legs.
I made these pants two years ago and have since gotten rid of any leftover scraps so I just grabbed some flannel from my stash and used a pair of his linen store bought pants as a guide for the gusset size. The section measured about 20" long and the widest section was 4". So, what I did was cut a rectangle 20"x4" and then folded it in half lengthwise and then widthwise and cut a curve to make it oval-ish in shape.
Using my trusty seam ripper, I opened up the inseam about 10" on each side and then forward/backstitched the four opened points as a necessary reinforcement to stop the seams from continuing to open.
Then I laid the gusset, which I overlocked to prevent fraying, in the new opening and pinned it in place. I stitched it in using a 3/8" seam allowance and bam. Now my BF can move about comfortably and continue to enjoy his homemade lounging pants, indefinetely. Unless you plan on wearing the garment only at home, I recommend finding fabric as close to that of which you are trying to insert a gusset into, for obvious reasons.
Looks a little wackadoodle.