Straight or Plain
A straight seam is made by placing the ‘right’ sides of the fabric together lining up the edges. Then the fabric is fed through the sewing machine under the presser foot with the desired seam allowance, which is how much space is left between the stitching and the cut edge.
True Flat Felled
A true flat-filled seem has interlocking edges of fabric and two sets of stitching going through four layers of the fabric. This is done with specialty equipment and machines that feed the fabric into the presser foot and make the fold.
With a bit of practice and some ingenuity, this can also be made on a straight stitch machine or a domestic home sewing machine. It is not as fast but it is just as strong. Flat-filled seams are often used on the inside inseam of pants so they do not split since they go through four layers of fabric.
Mock or Faux Flat Felled
A Faux Flat Felled seam is made starting with the wrong sides of the fabric together lining up the edges. Then press the seam to one side. Trim the lower seam allowance in half. Fold the top seam allowance around the bottom seam allowance enclosing it. Use an iron to press the seam allowance flat and around the bottom allowance. Make sure the width is even. Stitch a second set of stitches along this folded edge. This seam is not as strong as a true flat-felled seam that goes through four layers of fabric twice.
French Seam
A french seam is made by placing the wrong sides of the fabric together lining up the edges. Then stitch a 1/4 inch seam allowance. Press the fabric and then topstich.
Busted Seam
This seam is often seen on the outside of jeans. Sometimes it uses the selvage edge of the fabric. The selvage edge of the fabric is the factory edge that does not unravel. Other times it uses surged edges of fabric. It is a basic straight seam that then is pressed flat and open, or “busted” open.
Serged
An overlock stitch machine or serger is used to keep the threads from unravelling on the cut edge of woven and knit fabrics. A serger is ideal for stretchy fabrics since the the stitches will resist breaking when the fabric is stretched. Since a serger cannot backstitch, the chain from the serger needs to be secured so the seam does not unravel.