Stitch and Seam Classes: The ISO and ASTM Reference

A "seam" and a "stitch" are not the same thing, and the industry keeps them in separate standards for a reason. The stitch is the thread structure that holds material together. The seam is the geometry of how the material is positioned before the stitch goes through it. A single stitch type can build many different seams, and a single seam type can be sewn with several different stitches. Specifying a garment means picking one of each.

Two international standards define these systems, and one North American practice restates them in the letter codes most tech packs use.

Two standards, one system

The classification traces back to the United States Federal Standard 751a, "Stitches, Seams, and Stitchings," which assigned the numeric stitch classes and the two-letter seam classes still in use. That standard was withdrawn, and its content now lives in two places a working designer will cite:

• ISO 4915 covers stitch types and gives each the three-digit number (301, 401, 504, and so on).
• ISO 4916 covers seam types and gives each a class plus a configuration.
• ASTM D6193, "Standard Practice for Stitches and Seams," is the North American document that carries the same definitions and uses the familiar two-letter seam codes (SS, LS, BS, FS, EF, OS).

The numbers and letters are stable across all three. A 301 lockstitch is a 301 everywhere, and an SSa seam means the same thing to a pattern maker in Los Angeles and a sewing line in Da Nang. That portability is the whole point of learning the codes.

The six stitch classes

Every machine stitch belongs to one of six classes, set by the first digit of its three-digit number. The class tells you the thread structure. The remaining digits identify the specific stitch within the class.

Class 100: Single-thread chainstitch

One needle thread loops through the material and interloops with itself on the underside. It uses no bobbin, so it never runs out of bottom thread, which is why bag-closing and long basting operations use it. Its weakness is the same as its structure: pull the tail and the whole line can unravel in one motion. Type 101 is the basic single-thread chainstitch. Type 103 is the blindstitch used for invisible hems.

Class 200: Originated as hand stitches

This class covers stitches formed by a single thread passed through the material as one line, the machine descendants of hand sewing. Saddle stitching and pick stitching sit here. On production garments it appears mostly as a decorative or tailoring detail rather than a structural seam.

Class 300: Lockstitch

Two threads, a needle thread from the top and a bobbin thread from below, interlace in the middle of the material. Type 301 is the single most common stitch in apparel and the default for wovens. It looks identical on both faces, holds tight, and resists unraveling because the two threads lock at every stitch. Its limitation is stretch: a straight 301 seam has little give and can pop on a knit or a close-fit woven under strain. The bobbin also runs out, which slows a production line. Type 304 is the zigzag lockstitch used for attaching elastic and for stretch on wovens.

Class 400: Multi-thread chainstitch

One or more needle threads interloop with one or more looper threads underneath. Type 401, the two-thread chainstitch, is the workhorse for long, strong, load-bearing straight seams: jeans side seams, inseams, and waistband joins. It carries more stretch than a lockstitch and builds a stronger seam, and it has no bobbin to replace. If it fails it can run back from the broken end, so it is backtacked or crossed by another seam at each terminal.

Class 500: Overedge chainstitch

One or more needle threads plus looper threads that wrap around the edge of the material, trimming and finishing in the same pass. This is overlocking, or serging. Type 503 is a two-thread edge finish. Type 504 is the three-thread overlock that finishes a raw edge and stretches with a knit. Types 512 and 514 are four-thread mock-safety stitches. Type 515 and 516 are five-thread safety stitches that combine an overedge with a chainstitch to seam and finish at once. The class stops fraying and gives stretch, which is why nearly every knit garment is assembled on it.

Class 600: Covering chainstitch

The most complex class, using up to nine threads: needle threads, looper threads, and a top-cover thread that lies across the top surface. It covers both faces of the seam flat, so it neither adds bulk nor irritates the skin. Type 602, 605, and 607 are the coverstitches and flatlocks that hem T-shirts and leggings, attach binding and elastic, and join the butted edges of activewear. It is the stretchiest, flattest seam available and the reason a good athletic hem lies smooth against the body.

The six seam classes

Where stitch classes describe thread, seam classes describe geometry: how many plies of material are involved and how they sit relative to each other and to their edges. ASTM D6193 uses six two-letter classes. A lowercase letter is added for the specific configuration, and a following number gives the rows of stitching, so a code reads like SSa-1 or LSc-2.

SS: Superimposed seam

Two or more plies stacked face to face and seamed a set distance in from the edge, then opened out. This is the plain seam, the most common construction in all of apparel. SSa is the basic version. Sewn with a 301 lockstitch on a woven or a 504 overlock on a knit, it joins almost everything: side seams, shoulder seams, center backs. ISO 4916 files this as its Class 1.

LS: Lapped seam

Plies overlapped rather than stacked, so each lies partly on top of the other. The lap lets both raw edges fold inside the join. LSc is the flat-felled seam of a jeans inseam and a work shirt, sewn with two rows so both edges are enclosed and the seam lies flat and strong. Lapped seams also attach patch pockets, plackets, and yokes. ISO Class 2.

BS: Bound seam

An edge, single or multiple ply, wrapped in a separate strip of binding and stitched through. The binding finishes and covers the raw edge in one component. BS seams bind necklines, armholes, and the inside seams of unlined jackets, and a contrast binding turns the finish into a visible detail. ISO Class 3.

FS: Flat seam

Two edges butted together, not overlapped, and joined by stitching that spans the gap so the seam lies completely flat with no ridge. Sewn with a class 600 coverstitch or flatlock, it is the low-bulk, non-chafing seam of performance base layers and seamless-feel activewear. ISO Class 4.

EF: Edge finishing

A single ply whose edge is finished or neatened, with no second ply joined. A serged raw edge, a turned-and-stitched hem, and a narrow rolled hem are all EF. EFb is the common folded hem. This class is about treating one edge, not joining two panels. ISO Class 6.

OS: Ornamental stitching

Stitching worked on a single ply for appearance rather than construction: topstitch detailing, pintucks, decorative rows. The stitch holds nothing together structurally, it is there to be seen. ISO Class 5.

How a seam gets specified

A construction call-out names both systems at once. The seam class and configuration come first, the stitch type second:

SSa-1, stitch type 301 is a plain superimposed seam, one row, sewn as a lockstitch. That is a standard woven side seam.

LSc-2, stitch type 401 is a flat-felled lapped seam, two rows, sewn as a two-thread chainstitch. That is a jeans inseam.

EFb-1, stitch type 406 is a folded edge finish, one row of coverstitch. That is a knit T-shirt hem.

This is why a tech pack construction page lists both. A line that reads "side seam: SSa-1, 301, 10 to 12 SPI" tells the factory the ply arrangement, the stitch, and the density, and a sample sewn anywhere in the world comes back the same. Leave the codes off and the factory chooses, which is how a stretch knit ends up with a popped lockstitch seam it should never have had.

Choosing the stitch and the seam

The seam class usually follows from the garment and the panel. Side seams are superimposed, jeans inseams are lapped, activewear joins are flat, hems are edge finishes. The stitch class follows from the fabric's behavior and the load on the seam:

• Stable woven, ordinary seam: superimposed seam, 301 lockstitch. Secure and clean, and the low stretch is not a problem on a woven.
• Woven under load, or a long structural seam: lapped or superimposed seam, 401 chainstitch. More strength and no bobbin changes on a long run.
• Any knit seam: superimposed or flat seam on a 504 overlock or a class 600 coverstitch, so the seam stretches with the fabric instead of restraining it.
• Raw edge that will show or fray: edge finish, 504 overlock, or a bound seam where the inside is visible.
• Hem on a knit: edge finish, 406 or 602 coverstitch, flat and stretchy so it lies against the body.

Thread choice, stitches per inch, and seam allowance sit on top of these decisions, and all of them belong on the tech pack next to the codes.

Putting it on the tech pack

The value of the system is that it removes interpretation from the sewing line. A construction page that lists the seam code, the stitch type, and the stitch density for every operation gives a factory an unambiguous target, and a sample sewn to those codes is repeatable across suppliers and across production runs. If a garment relies on a specific construction to fit, wash, or wear correctly, that construction belongs in the standard's codes, not in a photograph the factory has to guess from.