A French seam, which is one of the most common seams found in apparel, can be used in pants, shirts, unlined bags, and dresses.
The mock French seam is used for similar purposes, but works better around curves than the French seam.
Sew the pieces of your project together with a 1/8th inch seam allowance, and press the seam to one side. Fold in half with the seam on the inside, and iron again.
Stitch down the fold with a 1/2 inch seam allowance, make sure to back stitch at the start and end.
After stitching your pieces together, iron the seam open. Then, iron each edge over 1/8th inch towards the middle.
Press the middle together to one side. Fold the piece in half with the seam on the outside and iron.