How a garment is held together is a bigger decision than it looks. The wrong seam on a knit pops on the first wear, the wrong seam on a sheer shows through, and the wrong seam anywhere adds cost the style did not need. The seam is a construction choice, not a detail, and it belongs in the spec.
This guide covers the seam types you actually choose between, which fabric and end-use each suits, and how a seam is really classified and specified in a tech pack.
What are the main types of seams?
In day-to-day use there are five seam types worth knowing: the plain seam, the overlocked or serged seam, the French seam, the flat-felled seam and the bound seam. The industry classifies them more formally under the ISO 4916 standard, into eight seam classes, but these five cover almost everything a designer specs.
A seam type is not the same as a stitch type. The seam is how the fabric pieces are arranged and joined; the stitch is the thread structure that holds them, from a plain lockstitch to an overlock. A tech pack names both, and getting the pair right is what makes a seam hold.
Why the seam type matters
The seam decides whether a garment survives wear, how it looks inside and out, and part of what it costs to make. Choosing it is a construction decision with real consequences.
- Strength and end-use: a high-stress seam on jeans needs a flat-felled seam and a chainstitch; the wrong choice fails at the crotch or the inseam.
- Fabric behaviour: a knit needs a seam and stitch that stretch, or the seam pops on the first wear; a sheer needs an enclosed seam or the allowance shows through.
- Appearance inside and out: an unlined jacket or an open-backed garment shows its seams, so the inside finish is part of the design, not an afterthought.
- Cost and speed: an overlocked seam is one fast pass; a French or flat-felled seam is two or three, and a bound seam adds material; the seam choice moves the labour minutes on every unit.
The main seam types and when to use each
Each seam suits a different fabric, strength and finish. This is the decision most of the guides skip, because they teach how to sew a seam rather than when to choose it.
- Plain seam: two pieces joined face to face with the allowance inside; the default for most wovens. Fast and cheap, but the raw edges need a finish, usually an overlock, or they fray.
- Overlocked (serged) seam: sews the seam, trims the edge and encloses it in one pass. The workhorse of knit garments and high-volume production, because it is fast and it stretches. Most of your t-shirts use it.
- French seam: encloses the raw edges inside the seam itself for a clean inside with no exposed allowance. Best on sheers, silks and delicate or unlined garments; it takes two passes and works only on straight or gently curved seams.
- Flat-felled seam: the strongest common seam, self-enclosed and locked down with one or two rows of topstitching. The seam of jeans, workwear and shirts, where it takes stress and looks clean on both sides; it is slower and usually wants a dedicated machine.
- Bound seam: the raw edge is wrapped in a strip of fabric or knit binding. Used on knit necklines in production and, as a Hong Kong finish, on unlined jackets and premium tailoring; it adds material and labour.
The industry system: ISO 4916 seam classes
Behind those everyday names is the formal system a factory and a tech pack use. Under ISO 4916, every stitched seam falls into one of eight classes, defined by how the pieces are arranged.
- Class 1, superimposed: pieces laid on top of each other and joined near the edge; the plain seam and the French seam live here. The most common load-bearing seam.
- Class 2, lapped: pieces overlapped and stitched; the flat-felled seam is the best-known lapped seam, used on jeans.
- Class 3, bound: an edge wrapped in binding; t-shirt necklines and Hong Kong finishes.
- Class 4, flat (butted): two edges butted together and stitched across, with no overlap; used on knits and underwear to keep the seam flat and free of bulk.
- Classes 5 to 8: decorative or ornamental stitching, edge finishing and neatening (serging), attaching separate items, and single-ply construction such as belt loops and hems.
The old four-class shorthand, superimposed, lapped, bound and flat, is still how many people describe seams, and it maps onto the first four ISO classes.
Seams, stitches and the tech-pack spec
A seam is only half the spec. The stitch type that sews it is classified separately, under ISO 4915, into six numbered classes, and the right stitch depends on the fabric.
- 301 lockstitch: needle and bobbin interlock, identical top and bottom; strong and stable on wovens, but it has almost no stretch, so it cracks on knits. The default topstitch and single-needle seam.
- 401 chainstitch: needle and looper; stronger and more elastic than a lockstitch, which is why it sews denim mainseams and the flat-felled seams of jeans.
- 500 overlock: wraps the edge with looper threads; high strength and high stretch, the standard for knit assembly and edge finishing.
- 600 coverstitch (flatlock): a flat, stretchy multi-thread stitch for knit hems and flat seams on underwear and activewear.
A seam is fully specified only when the tech pack names the seam class, the stitch type, the stitch density in stitches per inch, the seam allowance and the thread. Those are the variables seam engineering uses to match a seam to the fabric and the make.
How to choose and spec a seam
For a brand, choosing a seam is a fabric, strength and cost decision, and specifying it fully is what stops the factory from defaulting to whatever is fastest on their line.
- Match, then spec: match the seam to the fabric and end-use, a knit to an overlock or coverstitch, a sheer to a French seam, a high-stress seam to a flat-felled, an unlined premium garment to a bound seam, then name the seam class, stitch type, stitches per inch and seam allowance in the tech pack.
A few faults recur, and each traces to the wrong seam or stitch for the job.
- Seams that pop on knits: a rigid 301 lockstitch on a stretch fabric; use a stitch that stretches, an overlock or a coverstitch.
- Seam grin and slippage: the seam gaps open under tension; raise the stitch density and check the thread and the seam allowance.
- Puckering: tension or the wrong stitch for a fine fabric; adjust the stitch density and tension, or change the seam.
- Fraying inside: a plain seam left without an edge finish; overlock or bind the allowance, or choose an enclosed seam.
And approve a sewn seam on the actual production fabric, not a substitute. A seam behaves differently on jersey, poplin and silk, so the approved sample on the real cloth is the spec the factory has to match.
The seam is one line in a garment's construction story, tied to the fabric, the machine and the cost of every unit. Apshan structures that fashion knowledge, cited to source, so a design decision is grounded rather than guessed. Request access or see the plans and pricing.