A friend asked me if I could mend her holey jeans.  Made of stretch denim, they had worn through in the crotch.  This is unsurprising, since there's a lot of strain in a small area in the crotch of jeans.  Seeing that the fabric was thinning around the hole as well as at the point of damage, I decided to replace the whole crotch with a gusset.

In sewing, a gusset is the opposite of a dart: a dart shapes fabric by removing some; a gusset shapes fabric by adding some.  Gussets can be found in interesting places, like armpits and crotches.  In the early days of bras, before molded foam, the cups were made with gussets.  Corsets sometimes have hip gussets.  Basically, if the fabric is too tight and you slash it, that's a slit; fill the slit with fabric, and it's a gusset.  In this case, I decided to patch the hole, then cut away the original fabric, turning the patch into a gusset. 

A friend gave me this slip, which had sentimental value to her, because she thought I'd be charmed by its sweet colors and soft lace.  It was, however, a little the worse for wear.