Every woven fabric frays, some much more than others. A seam finish wraps, covers or binds the raw edge so the cloth cannot unravel in the wash. It also decides how the inside of the garment looks, which matters on an unlined piece.
Zigzag: the quick finish
A zigzag stitch over the raw edge, close to the stitching line. It is the fastest finish, it needs no special equipment, and it works on almost anything. It is also the least tidy, so it belongs inside a lined garment or a casual piece.
Overlock: the fast, stretchy finish
A serger trims the allowance and wraps the edge in thread in one pass, which is why it is the fastest finish of all. It also produces a stretchy seam that moves with a knit, so on jersey it is not just a finish but the seam itself. It needs a second machine and some patience with threading.
French seam: the clean one
A French seam encloses the raw edges inside a second seam, so nothing shows on either side. Sew the seam wrong sides together, trim the allowance, then sew it again right sides together to trap the raw edge inside. It is beautiful on sheer and fine fabrics, and it is the wrong choice on a heavy one, because it doubles the bulk.
Flat-felled: the strong one
A flat-felled seam wraps one allowance over the other and topstitches it flat. It is the strongest finish, it lies completely flat, and it is the seam on a pair of jeans. It is also the most work, and it needs a fabric that will press.
| Finish | Fabric | Bulk | Inside looks | Effort |
|---|---|---|---|---|
| Zigzag | Any woven | Low | Untidy | Very low |
| Overlock | Any, best on knit | Low | Neat | Low, needs a serger |
| French | Fine and sheer | Low | Perfect | Medium |
| Flat-felled | Denim, linen, cotton | Adds a little | Very neat | High |
Match the finish to the fabric
Fraying is the deciding factor. A stable cotton is fine with a zigzag; a loose linen or a sheer silk wants a French seam or a bound edge. If the fabric frays badly, the finish has to cover the raw edge completely.
Press the seam flat as sewn
Before you finish it, press the seam as it came off the machine. A finish applied to a wrinkled seam sets the wrinkles in.
Trim the allowance evenly
A finish is only as tidy as the edge under it. Trim the allowance to a consistent width first.
Apply the finish
Zigzag, overlock, enclose or fold, depending on the fabric and the garment.
Press the finish
Press the finished seam so it lies flat and the finish settles into the cloth.
Which to choose
On an unlined garment, use a French seam on fine fabric and a flat-felled seam on heavy fabric, because those are the two that look good from both sides. On a lined garment, a zigzag or an overlock is enough, because nobody will see it. On a knit, use the overlock, because the seam needs to stretch.
Sharp tools in this step
Trimming a seam allowance puts shears close to your fingers and close to the seam you just sewed. Trim in short strokes with the hand holding the fabric behind the blade, and never trim toward yourself.
The finish is not the interesting part of a garment, but it is the part that decides whether the garment survives a year of washing.
