Ooh, here's a good question that came from a commenter. Have you ever noticed how the terms gathering, ruching, and shirring are often used interchangeably? Well, it's probably because there's a lot of overlap between these three methods. I'm going to talk about what I understand these techniques to be, but feel free to jump in with your own opinions in the comments, as always!
This term refers to a length of fabric being drawn up into gathers and sewn into a shorter length of fabric. The best example of gathering in vintage style is a dirndl skirt, which is a rectangular pattern piece that is drawn up to fit into a smaller piece, like a waistband or bodice.
Ruching is a gathered overlay. The fabric is gathered on two parallel sides and stitched to an underlay. It's often done in sheers, like chiffon.
Here's a bodice that has a ruched chiffon overlay.
Ruching is made by creating two or more parallel lines of gathering.
Shirring is a gathering technique done with elastic thread, which creates a stretchy garment. Elastic thread is used in the bobbin, causing the fabric to gather up when stitched. Shirring is very common on the side panels on 50s swimsuits, rompers and sundresses a la Alfred Shaheen.
I love the form-fitting effect shirring has on this Shaheen romper.
There you have it: gathering, ruching, and shirring as I understand them. Please share your own definitions!