Stay stitch can be defined as a row of regular machine stitches, done on a single thickness of fabric between the cut edge and the seam line, about 0.2cm from the seam line. This preserve the lines and grain of the fabric as cut by the pattern. Generally it is done on curved edges like neckline, armhole line and collar patterns.
When patterns are cut on to the fabric depending upon the nature of the fabric, any edge that is cut on the bias line will stretch. This stretching can be noticed particularly in crepes and other heavy, slippery fabrics. It happens to some extent on all fabrics except like taffeta and canvas. The purpose of stay-stitching is to keep the bias edges of a garment from ravelling in other words, to ensure the exact measurements of the pattern.
One of the characteristics of lengthwise and crosswise threads of fabric is that they will not stretch; however, any slightly diagonal line is a bias line and will exhibit characteristics stretching. Such bias edges of a garment include: front and back armholes, front and back shoulders, front and back neck edges, and many other edges, depending on the design. However, there are exceptions-edges which need not be or should not be stay-stitched. Seam edges which will fit the body snugly must be stay-stitched; edges which will hang free from the body need not be stay-stitched. Stay stitching saves time for beginners.
There are several edges which need not be stay-stitched. They are:
Edges cut on the lengthwise or the crosswise threads.
Outer edges of facings (unnotched edges).
The cap of the sleeve.
How to stay-stitch: As soon as the garment is cut, either just before or just after the tailors tacks are made, remove one pattern piece at a time and prepare to stay-stitch the necessary edges.
Stay-stitches are made through only one thickness of fabric. The stitches are done by machine with a stitch long enough to pull up if it becomes necessary to draw in the fabric to make it return to the size of the pattern. At the same time the stitch must be short enough to serve the purpose of holding the edge firmly. In most fabrics, except the heaviest of wools, this stitch should be about 1/10 inches long (10 stithes per inch). In heavy wools, the stitch might have to be 1/8 inch long (8 inches per inch) in order to be long enough to pull up.
Use thread that matches the garment . The reason for this is that stay-stitches do not need to be removed after the garment is finished and a matching color will make the garment look more attractive from the inside.
The stay-stitches must be placed outside the seamline, in the seam allowance. They may be placed any where from 1/8 to 1/12 inch from the cut edge. Most authorities give 1/8th inch as the correct placement because of this advantage: if you should have to let out the seams of the garment, these stitches would not show and would not have to be removed. The 1/8th inch measurement works very well on fabrics that are too limp and slippery. Stitches 1/2 inch from the cut edge have the great advantage of giving control very close to the seam line, where control is badly needed at the seam line. Since the stitching done through one thickness of the fabric and since the machine is keyed to stitching through at least two thicknesses of fabric, and since a longer stitch than usual is being used, the fabric is apt to pucker. This can be counteracted by a firm gentle pull on the fabric as it comes through the machine. How much to hold it and whether to pull it slightly depends on the fabric. To find out, stay-stitch one edge of a piece, holding the material so that it comes out of the machine with no puckers. Then lay the pattern piece back on the fabric and see if that stay-stitched edge is just like the pattern. If the fabric is now shorter than the pattern, then hold it more firmly as it moves through the machine; if the fabric is longer than the pattern, hold it less firmly.
Stay-stitch each edge of the fabric with separate threads, leaving just short ends of thread at each corner.
Always put every pattern piece back on the fabric and check every edge to see that it has retained the same measurements as the paper pattern. If an edge is too long, pull up the bobbin thread until that edge measures exactly the same as the pattern. If an edge has been drawn up and is too short, loosen the stitches and break them if necessary in order to achieve an exact copy of the pattern. (Figure).