Stay stitching is a single line of stitching, through one layer of fabric, that is placed to stabilize the fabric, preventing it from becoming stretched or distorted.
Where It Is Used
This type of stitch is usually called for on the edge of a piece of fabric that has a bias cut to it which would allow the fabric to easily become distorted. See more about bias-cut fabric below.
Sewing the stay stitching in a certain direction, referred to as directional stitching, will keep the shape of the fabric as it was cut out of the fabric. Sewing the stay stitching opposite of suggested directional stitching can distort the pieces and counteract the reason for stay stitching.
The same stitching width and length that you will be using to construct the garment can be used for stay stitching.
Don't Skip This Step
Although it may seem like you are accomplishing nothing and it's a step you can eliminate, if pattern directions call for stay stitching, do it!
You might think you are saving yourself some time by skipping this step, but in reality, you are preventing all kinds of problems in the later steps of putting the pieces together.
Fabric that hasn't been stay stitched is very apt to become distorted while you press the pieces, add interfacing to the fabric or even just simply handle the cut-out fabric.
When you skip the stay stitching process, by the time you start to join the pieces of fabric, they may be so out of shape that you have a battle on your hands to make the pieces fit together. Taking the few minutes it requires to stay stitch the cut-out pieces of fabric, eliminates problems later in having collars, facings and other pieces of the pattern fit together.
Use Your Judgment
Not all skirts or dress patterns will tell you to stay stitch. But the more loosely woven your fabric is, the more likely the threads of the fabric will move and distort the original cut-out fabric. When in doubt, stay stitch.
There are three types of fabric grain -- lengthwise grain, crosswise grain, and bias grain.
Bias grain refers to the thread line that is at a 45-degree angle to the lengthwise and crosswise grain of fabric as it is on the bolt. The bias has stretch in woven fabric and will hang differently than a garment that has been cut on the lengthwise or crosswise grain.
Example of When to Stay Stitch
The neckline edge of a garment is cut on the bias but the collar is on the straight grain. It will be necessary to stay stitch the neckline for the collar to fit properly.