This tutorial was featured on the Oliver + S blog.
A flat-felled seam is accomplished by sewing two lines of stitching at the seam while simultaneously enclosing the raw edges of the fabric. This seam finish not only prevents unraveling or fraying, but makes the seam very strong. It also gives the finished garment a clean finish on this inside. A flat-felled seam is commonly found on denim jeans and men's dress shirts as well as on reversible garments. I've even seen this feature instructed in vintage patterns for boy's and men's pajamas. It adds one more touch of professionalism to a hand-made garment.
To begin, stitch the seam with the fabric pieces wrong sides together. You can determine how wide you want your flat-felled seam to be by how much you trim your seam allowances. Begin by trimming only one side to the desired width of the finished seam allowances.
Trim the second seam allowances to twice the width of the first. For this example, I stitched a ½” seam and then trimmed the first seam allowance to ¼”, which means I didn’t need to trim the second seam allowance since it was already twice the width of the first.
Fold and press the wider seam allowance around the shorter seam allowance so the raw edge of the second (wider) seam allowance meets the first seam line. Then stitch along the folded edge of the second seam allowance. This encloses the raw edge, and with the second row of stitching creates a very strong seam.
To make my stitching rows evenly spaced, I used a 1/4-inch foot. The black guide on the right followed the first stitching line while the inside edge of the little toe was a guide for sewing the second stitching line.
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Another useful tool is the Felling Foot, because it folds, presses, and stitches the first row of stitches all in one pass. Using this attachment, while still accommodating a 1/2-inch seam allowance, I trimmed 1/8 inch from each of the seam allowances prior to stitching so that the finished seam would finish with the correct seam allowance according to the pattern.
To begin, space the top seam allowance 1/8 inch to the left of the bottom seam allowance as shown in the photo above. Finger press and fold the start of the seam so that you can stitch 2-3 stitches to secure the folds in place. With the needle still in position, maneuver the bottom seam allowance into the Felling foot so that it curves up and around - sewing slowly, carefully begin stitching to allow the bottom seam allowance to fold over the top seam allowance so that the folds can be stitched in place for the first stitching line.
Simultaneously, the attachment is folding, pressing and allowing the needle to stitch the initial seam enclosed. Press the enclosed seam so that you can make the final pass with the attachment.
Using the Feller attachment for the second pass, slip the stitched fold up into the attachment with the right toe set up against the outside of the first row of stitches. Allow the fold to carefully pass under the foot while stitching it in place for the finished Flat-Felled Seam. The 4mm Feller Attachment makes a dainty, 1/8" narrow flat-felled seam.