Definition
Use a basting stitch to temporarily hold two pieces of fabric together or to gather fabric into ruffles. The stitch should be very long, a half inch for light fabrics, and often the sewer will run the needle through the fabric three or four times before pulling the string through. Unless basting is used for making ruffles, the thread should be left with a little give; for creating ruffles, the thread should be taught. Hand basting is sewn with a running stitch because a running stitch is easily removed.
Example
When a seamstress is making a dress, they will often baste the seams first and then, after the client tries on the dress, pin the new seams and stitch those. They then remove the original basted seams.
Benefits
The benefits of basting include being able to complete a garment without worrying about the perfect fit until after a fitting. Basting, when working with slippery or very stretchy fabrics would also help hold a form better than simply pinning. Another benefit is that it can be done on the go: hand basting a ripped cuff or seam while at work and sewing more accurately when returning home.
Sewing Machine Tips for Basting
On a machine, the longest stitch setting is used for basting, usually this will be a a 6 to 8 stitch. Use pins, even though the stitches will be taken out eventually. Using pins the first time will lead to a better, cleaner look and help practice pinning for beginners. Do not back stitch at the beginning and end of the seam, this will only make pulling out the stitch more difficult. Also, do not sew directly on top of the basted seam, this will make the original stitch harder to pull out, and weaken the new tighter seam.
Tips
The tighter you want the pieces of fabric held together, the smaller the baste stitch should be, but always remember the seam should never be stronger than the fabric. When working with heavy duty fabrics, stitch around 3/8", and keep the string tighter. Even when using a smaller stitch, each individual stitch will be about 1/4" apart.