For most piecing, good-quality cotton or cotton-covered polyester is the thread of choice. Inexpensive polyester threads are not recommended because they can actually cut the fibers of cotton fabrics.
Use the best quality thread you can find. Inferior cotton threads create a lot of lint, especially when machine-sewing. This lint gathers in the sewing machine's moving parts and creates problems. Avoid bargain threads. They are not usually durable in the finished product.
Choose thread colors that will match or blend with fabrics in your quilt. If using dark fabrics, a dark gray would be a good choice. If using light fabrics, off-white blends well. When making scrap quilts of many colored fabrics, choose a neutral thread, such as medium gray.
The threads should not be seen when pulling at seams. If using a light thread when sewing dark fabric patches, the thread will probably show when the seam is pulled. Test a sample seam to prevent this problem.
The same threads used for piecing can be used for quilting, but they need to be waxed to keep them from tangling when quilting. Quilting threads, made specifically for quilting, may be purchased in a variety of colors. Quilting thread reduces tangling and adds strength to quilting stitches.
For machine appliqué, rayon threads add luster to the stitches. For hand appliqué, use colors that match or blend with each piece to be appliquéd. This may mean changing threads often.
Do not use quilting thread for machine quilting unless the thread is labeled as machine-quilting thread.
Nylon monofilament thread is available in clear for light backgrounds and smoke for darker backgrounds. It is recommended by some for machine quilting.
Specialty threads such as metallics are sometimes used for quilting. Improvements have been made in these threads in recent years to prevent breakage when stitching. These threads work well for machine quilting and add luster to your finished project.