What is a seam?
A seam is a line of stitching that joins two or more layers of fabric. The strip of fabric between the stitching and the cut edge is the seam allowance. It is usually 1.5 cm in clothing patterns, and 1 cm or less on some knitwear.
The cut edge is called the raw edge. Woven fabric frays at a raw edge, so most seams also neaten it (zigzag, overlocking, pinking or binding).
Try it: find a seam inside a T-shirt and a pair of jeans. Count the rows of stitching.
Common seam types and how to choose
- Plain (open) seam: right sides together, one row of stitching, allowance pressed open. Quick and flat. Used on most woven garments.
- French seam: sew wrong sides together first, trim, fold, then sew again with right sides together. The raw edge is trapped inside. Best for sheer or fine fabrics such as chiffon and silk, and for unlined items.
- Flat-felled seam: one allowance is folded over the other and stitched down, giving two visible rows. Very strong and fully enclosed. Used on jeans, shirts and workwear.
- Overlocked seam: an overlocker stitches, trims and wraps the edge in one go. Fast and stretchy; standard on T-shirts and knit fabrics.
- Bound seam: the edge is wrapped in bias binding; neat for unlined jackets.
Choose a seam by: fabric weight and fray, stretch, the strength needed, whether the inside will be seen, comfort on the skin, speed and cost in mass production.
Threads and fusible layers
Thread holds the seam. Match the thread to the fabric and the job:
- Polyester: strong, slightly stretchy, does not rot; good all-purpose thread.
- Cotton: matches cotton fabric and takes dye the same way, but is weaker.
- Core-spun (polyester core wrapped in cotton): strong and smooth; used in industry.
- Nylon or heavy polyester: for upholstery, bags, outdoor gear. Elastic thread for shirring.
Interfacing is an extra layer that gives shape and support to collars, cuffs, waistbands and button stands. Fusible interfacing has heat-activated adhesive on one side: heat, steam and pressure from an iron or press bond it to the fabric. It can be woven, knitted or non-woven. Sew-in interfacing is stitched instead and suits heat-sensitive fabrics.
Joining without thread: thermoplastic synthetic fabrics can be heat-sealed or ultrasonically welded (waterproof seams in rainwear), and some items are bonded with adhesive or seam tape.
Fastenings and other components
- Buttons and buttonholes: neat, decorative, for shirts and coats; slow to do up.
- Zips: long, quick opening; concealed zips for dresses, open-ended zips for jackets, heavy metal zips for jeans.
- Press studs (snaps): quick and hidden; good for baby clothes.
- Hook-and-loop (e.g. Velcro): very easy for children, older people and shoes; can snag knits.
- Hooks and eyes, toggles, drawstrings, buckles, eyelets.
Other components: elastic, trims and braids, labels, shoulder pads, linings and interlinings.
Choosing components: think of the user (age, ability), function (how often it opens, load), safety (small parts can be a choking risk for babies; no drawstrings at a young child's neck), care (must survive washing), compatibility with the fabric weight, look, cost and sustainability.
Key formulas and definitions
- Seam allowance (standard) = 1.5 cm from the raw edge
- Plain seam: 1 row, open · French seam: 2 rows, edge enclosed · Flat-felled: 2 visible rows, strongest
- Fusible interfacing bonds with heat + steam + pressure
- Fastenings: button, zip, press stud, hook-and-loop, hook and eye, toggle, drawstring
- Choose by: fabric, function, user, safety, care, look, cost
Worked examples
1. A pattern piece is 40 cm wide including a 1.5 cm seam allowance on each side. How wide is it after sewing?
Remove both allowances: 40 − 1.5 − 1.5 = 37 cm finished width.
2. Which seam would you choose for a chiffon scarf-style blouse, and why?
A French seam. Chiffon is sheer, so a raw or overlocked edge would show through. The French seam encloses the edge, looks neat from both sides and stops fraying.
3. Choose a fastening for a toddler's jacket and justify it.
A chunky plastic open-ended zip with a covered top, or large press studs. They are quick, need little finger skill and are firmly attached. Avoid small buttons (choking risk) and drawstrings at the hood (strangling risk).
4. A shirt collar goes floppy after washing. What component was probably missing or wrong?
The interfacing. A fusible woven interfacing of the right weight should be bonded to the collar with heat and pressure so it keeps its shape after washing.
Common mistakes
- Thinking a French seam is the strongest seam. The flat-felled seam is stronger; the French seam is chosen for neatness on fine fabric.
- Forgetting the seam allowance when working out finished sizes.
- Using fusible interfacing on fabric that melts or marks with heat (some synthetics, velvet). Use sew-in instead.
- Picking a fastening only for looks and ignoring the user — for example small buttons on baby wear.