Add Stiffness and Shape to Clothing and Costumes

Stop a moment and think about the top of a sleeve. How did flat fabric obtain a curve and become a sleeve? The flat piece of fabric that became a sleeve was cut with a curve and eased into the straight opening on a garment. There are many areas of a garment that fabric is eased to make it fit into another piece of fabric. Easing is achieved by drawing the fibers of the fabric closer together than they were woven without creating any tucks in the fabric.
Gathering fabric gives the fabric more fullness and poof than easing fabric can achieve. To gather fabric, rows of basting stitches are sewn in the seam allowance and then the threads are pulled to bunch the fabric. The "bunching" is done in a way that the fabric is evenly gathered. There are no tucks or folds in a gathered seam. The amount the fabric is gathered is determined by the desired effect and the type of fabric. A lightweight cotton fabric will gather much more than a heavy drapery fabric.
Gathering is usually done to achieve having the fabric poof and stand out. If the fabric is limp and does not stand out, attaching interfacing or tulle to the seam area will help the gathering stand out.
More about Ease and Gather