Gather (sewing)

Gathering is a sewing technique for shortening the length of a strip of fabric so that the longer piece can be attached to a shorter piece. It is commonly used in clothing to manage fullness, as when a full sleeve is attached to the armscye or cuff of a shirt, or when a skirt is attached to a bodice.
In simple gathering, parallel rows of running stitches are sewn along one edge of the fabric to be gathered. The stitching threads are then pulled or "drawn up" so that the fabric forms small folds along the threads. 
Gathering seams once involved tedious hand sewing of basting, which was time consuming, especially with heavy fabric. However, finer gathers could be achieved. Now, a quick and easy way to make a gather is to use a wide zigzag stitch with a sewing machine. Both the upper and lower thread are pulled long and placed in front of the sewing machine. Then zigzagging is carefully sewed over top of the two threads without catching the threads as it is sewn. At the end the thread is pulled and is then gathered. 