The raw edge of the seam allowance is encased, then topstitched on the outside.
Most fabrics. Items that need durable seams.
Makes a neat, clean finish. Raw edges will not ravel because they are completely encased. Makes a very sturdy seam.
Requires precise pressing and sewing. Somewhat time consuming. Makes a topstitched seam on the outside you may not want.
How to Make
Place your fabric pieces wrong sides together and stitch. Press the seam allowances open. Press the edge of one side of the seam allowance under 1/4”. Trim the other side of the seam allowance to 1/4”. Fold the folded seam allowance over the trimmed seam allowance. Press. Stitch along the edge of the folded seam allowance.