Shirring is a sewing technique which uses multiple rows of stitches with elastic thread to create rows of gathers. This technique can be used decoratively or functionally in sewing projects, and is related to smocking, an older embroidery technique. Both are designed to allow fabric to stretch and snug up to fit, allowing people to do things like creating necklines which will fit snugly while the garment is worn, and stretch to allow the wearer to pull the garment off.
Decoratively, shirring can be used to add structure and texture to a garment. It may also be used to add form when a garment is loose and may be a bit shapeless. For example, a shirred waist can be created so that a garment will pull in at the waistline and look more visually interesting. Shirring is also commonly used on tops to allow them to fit snugly on the chest and flare out over the abdomen. Necklines and sleeves can also be done with shirring so that they will fit snugly, without sliding.
Sewing shirred garments is generally done on a sewing machine. Elastic thread or specialized shirring thread can be used to do shirring, with the elastic thread going on the back of the garment, while regular thread is used for the upper part of the stitches. Sewers simply run the garment through the sewing machine to create several rows of stitches, and then spritz it with water and touch it with an iron to encourage the stitches to pull tight into multiple rows of gathers.
Shirring can take a bit of practice. If the stitches are too big, the gathers may feel loose and the shirred area will not lie flat and smooth. Stitches which are too small can also cause problems, including shirring which is too tight. Individual settings on sewing machines vary, and some actually have a shirring setting which can be used to make shirred garments, curtains, and other projects. Zig zag stitching, if it is available, can also be used.
Smocking is a much older sewing technique, dating to the Middle Ages. With smocking, stitches are used to gather fabric into a series of pleats, or are drawn over a series of pleats, depending on the style. No elastic is used; the pleats themselves are designed to expand and contract to allow for flexibility while fitting snugly in place. Several different embroidery stitches can be used for smocking, and some sewing machines are capable of doing smocking. Today, garments which are really shirred are often sold as "smocked," creating confusion about the difference between these two techniques.