Buyer guide
How a stitch differs from a seam, the stitch types a tech pack names, where reinforcement belongs, and the defects a specification can prevent.
In short
A stitch is how thread interlocks; a seam is how fabric pieces are joined by one or more rows of stitches. ISO 4915 sorts stitches into classes (lockstitch, chainstitch, overedge, covering chainstitch) and ISO 4916 sorts seams (superimposed, lapped, bound, flat). A tech pack should name the stitch type, the seam type, the stitch density and where reinforcement goes.
8 min read
Written by the AHM International product development team
Updated October 2026. Every worked figure is checked by automated tests.
Stitch and seam are not the same thing
A stitch is a single unit of thread interlocked with itself or with other threads. A seam is a joint between pieces of fabric, made by one or more rows of stitching in a defined arrangement. Two garments can use the same stitch in different seams, and one seam type can be sewn with different stitches.
Two international standards give the vocabulary a tech pack uses. ISO 4915 classifies stitch types by number: the 100 class (single-thread chainstitch), 300 (lockstitch), 400 (multi-thread chainstitch), 500 (overedge) and 600 (covering chainstitch). ISO 4916 classifies seam types, including superimposed, lapped, bound and flat seams. Naming the number removes the ambiguity that adjectives like "strong" or "flat" leave.
Common stitches
What each does best. Stitch numbers are ISO 4915 types.
| Stitch | Type | What it does | Where it is used |
|---|---|---|---|
| Lockstitch | 301 | Two threads lock inside the fabric, so both sides look the same. Neat, but a broken stitch can unravel | Topstitching, shirts, aprons, woven seams |
| Double chainstitch | 401 | Two threads form a loop chain on the underside. More stretch and often more strength than lockstitch, and a seam that can run if it is cut | Side seams of woven trousers and jeans, workwear |
| Three-thread overlock | 504 | Trims the edge and encloses it in thread in one pass | Edge finishing, light seaming of knits |
| Safety stitch | 516 | An overlock with a parallel chainstitch, a stronger seam that is finished in one pass | Woven seams in workwear |
| Coverstitch | 602, 605 | Flat, stretchy stitching with thread on both sides | Hems and neckbands on T-shirts and knits |
| Flatlock | 607 | A flat seam, with no bulk of seam allowance | Sportswear, seams against the skin |
Common seams
| Seam | How it is made | Why it is chosen |
|---|---|---|
| Plain (superimposed) | Two pieces laid face to face and stitched together at the edge | The simplest and most common join, often finished with an overlock |
| Felled (lapped) | Edges folded and lapped so the raw edges are enclosed, stitched in one or two rows | Strong, flat and clean; used on workwear, jeans and shirts that are washed hard |
| French | A seam enclosed inside a second seam, with no raw edge showing | A tidy finish on fine fabrics, where the raw edge would fray |
| Bound | A raw edge wrapped in a strip of fabric or tape | Finishes an edge, and hides it, on unlined jackets and aprons |
| Flat | Edges butted or overlapped and stitched with no seam allowance | Smooth against the skin and no bulk, for knitwear and sportswear |
Reinforcement: bartacks and stress points
Some points on a garment take far more strain than the rest: pocket corners, belt loops, strap ends, the base of a zip. A bartack is a short, dense block of stitching laid across such a point, and a back stitch at the start and end of a seam locks the thread so it cannot pull out. On an apron the points are pocket corners and mouths, and where the neck strap and waist ties are attached.
Reinforcement belongs in the tech pack as positions, not as an adjective: which points, which stitch, and how many stitches in the bartack. A garment described as "reinforced" without positions is reinforced wherever each sewing operator decides.
What a specification should state
- Stitch type
- The ISO number for each operation, or a clear description and a reference sample.
- Seam type
- Which seam joins which pieces, and which are finished with an overlock or bound.
- Stitch density
- Stitches per inch or centimetre for each operation. Too few and the seam is weak; too many can pucker light fabric.
- Thread
- The fibre (polyester thread is common for durability) and the thread size, matched to the fabric weight and the washing it must survive.
- Seam allowance and topstitch width
- Measured in millimetres, with a tolerance, so each lot matches.
- Reinforcement
- The positions and the stitch used for each bartack and for any back stitch.
Common sewing defects
- Skipped or broken stitches
- Gaps in the seam. A needle problem or thread tension, and a reason for a seam to open in the wash.
- Puckering
- The fabric gathers along the seam. Caused by tension, thread that is too heavy, or fabric that is too light for the stitch.
- Uneven stitch density
- The same seam sewn at different densities across a lot.
- Open or slipped seams
- Yarns pulling out of the fabric beside the stitching, common on slippery or loosely woven fabrics.
- Loose thread ends
- Untrimmed ends, a finishing defect that is easy to spot and to prevent.
Related questions
Lockstitch uses two threads that lock in the fabric and looks the same on both sides, but can unravel if broken. Chainstitch forms a chain of loops on the underside and is stretchier, though a cut seam can run.
A short, dense block of stitching across a stress point such as a pocket corner or strap end, to stop the seam pulling out.
A stitch that trims the fabric edge and encloses it in thread in one pass, used to finish edges and to seam knits.
It depends on the fabric, thread and garment, so it is agreed in the tech pack and checked against the approved sample rather than set by a general rule.
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