What Is an Overlock Stitch?
An overlock stitch is a looped stitch that wraps around the raw edge of fabric, trimming it with a small knife and finishing it in the same pass. It is the workhorse seam of knitwear: it joins panels, stops the edge from fraying or curling and stretches with the fabric. The machine that produces it is called an overlocker in the UK and a serger in the US, and the stitch itself is also called an overedge stitch.
The stitch works by interlocking loops from the needle thread or threads and the looper threads. Unlike a lockstitch, which locks two threads in a single line through the fabric, an overlock stitch forms loops that run over the edge, so the seam has built-in stretch. This is why it is the default for jersey, fleece and French terry, where a rigid lockstitch would pop when the garment stretches.
The class numbers above come from ISO 4915, the international standard that groups stitch types by structure. They are shown as commonly described, and your factory's own documentation is the authority for a particular seam. The sections below explain each stitch in turn, starting with how to read the classes.
ISO 4915 Stitch Classes in Plain Language
ISO 4915 sorts stitches into classes by how the threads interlace, and gives each stitch a three-digit number. You do not need to memorise them, but knowing a few helps you read a factory's spec and avoid mismatched expectations. The first digit shows the family: 300 for lockstitch, 400 for multi-thread chain stitch, 500 for overedge and 600 for covering chain stitch.
Common ISO 4915 classes in knitwear
| Class | Common name | Threads | Typical role |
|---|---|---|---|
| 301 | Lockstitch | 2 (needle and bobbin) | Topstitching, woven seams; low stretch |
| 401 | Two-thread chain stitch | 2 (needle and looper) | Jeans seams and some knit seams; stretchier than lockstitch |
| 504 | Three-thread overlock | 3 | Edge finishing and light seaming |
| 514 | Four-thread overlock | 4 | Main seam on tees, hoodies and joggers |
| 516 | Five-thread safety stitch | 5 (chain 401 plus overlock 504) | Stronger seams for woven and heavier garments |
| 602 | Two-needle covering chain stitch | 4 | Twin-needle coverstitch hems |
| 605 | Three-needle covering chain stitch | 5 | Wide coverstitch; flatlock-style seams and hems |
| 607 | Four-needle flatseam | 6 | Flat seams in underwear and sportswear |
Class numbers and names are as commonly described in the trade. Confirm exact classes and thread counts with your factory, as machine setups differ.
Not every factory refers to the class number; many simply say "4-thread overlock" or "twin-needle coverstitch". Both are acceptable in a spec as long as you and the factory mean the same thing, which is why a sample and a close-up photo are worth attaching.
Coverstitch: The Hem Stitch for Knits
A coverstitch is a stitch with two or three parallel rows of straight stitching on the top face and a looped, interlaced cover on the reverse. It does not trim or wrap the raw edge, so it is used where the edge has already been turned, such as on t-shirt hems, sleeve hems and neckbands. The rows of straight stitching look neat and echo a topstitch, while the looper on the back stretches with the fabric.
This is the stitch that gives a t-shirt hem its familiar twin-line finish. A single-needle hem on jersey would be a lockstitch, which is rigid and tends to pop when stretched. The coverstitch gives a flexible hem that moves with the body and recovers well. Our guide to hem types shows how this stitch compares to other hem finishes.
Coverstitch is also used for attaching binding, taping neckline seams and decorative topstitching on joggers and hoodies. Its limitation is that it needs a clean, turned edge to work on, and the reverse loops can catch if the garment is pulled hard.
Flatlock: Flat Seams for Comfort and Sport
A flatlock seam is formed by butting or slightly overlapping two raw edges and joining them with a wide covering stitch, so the seam lies flat with no ridge. The usual covering stitch classes, such as 605 or 607 as commonly described, create a ladder-like pattern on one side and a loop pattern on the other. Because there is no bulky seam allowance, flatlock reduces chafing, which is why it appears in underwear, activewear and some baby clothes.
Flatlock is also used decoratively. Placing the seam on the outside gives a deliberate ladder effect that works as a design feature on sports jerseys and streetwear. Flatlock seams can be a little less strong than an overlock stitch because the join relies on a narrow overlap, so check seam strength on stretch fabrics with a test. For technical sportswear built around flatlock seams, our sportswear manufacturer guide explains where this work sits relative to our core range.
Overlock Stitch vs Coverstitch vs Flatlock Compared
Choosing between these stitches usually comes down to the job: joining, hemming or minimising bulk. The table below sets the three side by side for the properties a buyer is most likely to ask about.
Overlock stitch, coverstitch and flatlock side by side
| Feature | Overlock | Coverstitch | Flatlock |
|---|---|---|---|
| Main job | Joins and finishes raw edges | Hems and attaches turned edges | Joins edges flat with no ridge |
| Appearance | Looped thread wrap over the edge | Parallel rows on top, loops under | Ladder on one side, loops on the other |
| Stretch | Good | Good | Good |
| Seam bulk | Moderate; a ridge from the seam allowance | Low; sits on a turned hem | Lowest; seam is flat |
| Edge finishing | Yes, encloses and trims the edge | No, edge must be turned first | Edge is butted or overlapped, then covered |
| Typical garments | Tees, hoodies, joggers, fleece | T-shirt and sleeve hems, joggers hems | Underwear, activewear, sport jerseys |
| Main limitation | Can show as a ridge on the skin | Needs a turned edge; reverse loops can snag | Narrower join, less seam strength |
General comparison. Strength and stretch depend on thread, stitch density and fabric, so test on your fabric.
Indicative stitch comparison
1 dot = low, 5 dots = high
| Overlock (4-thread) | Coverstitch | Flatlock | |
|---|---|---|---|
| Seam stretch | ●●●●● | ●●●●● | ●●●●● |
| Seam strength | ●●●●● | ●●●●● | ●●●●● |
| Flatness on skin | ●●●●● | ●●●●● | ●●●●● |
| Edge finishing | ●●●●● | ●●●●● | ●●●●● |
| Speed and cost | ●●●●● | ●●●●● | ●●●●● |
Indicative ratings based on typical stitch behaviour; actual results depend on threads, density and fabric.
In a typical hoodie, the shoulder and side seams use a four-thread overlock stitch, the hem and cuff attachments use overlock or coverstitch depending on the design, and the neck tape may be coverstitch. The same logic applies on sweatpants, where our sweatpants and joggers guide covers waistband and cuff seams.
Thread, Needles and Stitches Per Inch
Thread choice affects seam strength, comfort and appearance. Polyester core-spun thread is the usual choice for seams that need strength with stretch. Textured or woolly nylon is often used in the loopers of underwear and activewear for a soft feel against the skin. Spun polyester and cotton-wrapped options are used where a natural look is wanted. Always make the thread weight and colour a part of the specification, not an afterthought.
Stitch density is measured in stitches per inch, or SPI. More stitches per inch generally means a stronger, tighter seam but also more risk of puckering and a stiffer feel, especially on thin jersey. Fewer stitches create a more elastic seam that may be weaker. The ranges below are commonly cited working figures for discussion and vary by fabric, thread and factory.
Commonly cited SPI ranges by stitch
Stitches per inch (SPI), commonly cited
Typical, varies by product, thread and factory. Use as a conversation starter, not a specification; always confirm with the factory.
Needle choice is the other half of the equation. Knit fabrics need ballpoint or light ballpoint needles that push between loops rather than cutting them, because a sharp needle can cut yarns and cause holes or skipped stitches. Match needle size to fabric weight and thread, and change needles on a schedule, because a blunt needle is a common cause of skipped stitches and seam damage.
Common Overlock Stitch Faults and How to Prevent Them
Most seam failures are visible on the first sample, if you know where to look. The following list sets out the usual faults on knit seams, their causes and the standard fix.
- Skipped stitches: the loop was not caught, often from a blunt or wrong needle, or incorrect timing. Change the needle, check threading and fit the right needle type for stretch fabric.
- Popped seams: the stitch cannot stretch as far as the fabric, usually from a stitch with too much tension or too few stitches. Reduce tension, adjust density or choose a stretch-friendly stitch.
- Tunnelling: the fabric pleats between the rows of a coverstitch, caused by tight bobbin or looper tension, or loose fabric control. Rebalance tension and use the correct feed.
- Seam pucker: the seam draws up from uneven feed or over-tight threads. Adjust differential feed and thread tension, and test on offcuts.
- Wavy hems: the hem stretches while it is stitched. Use stable feed, tape or a binder, and avoid pulling the fabric through.
- Open seams at stress points: the seam fails at the armhole or crotch. Use a stronger stitch type, a five-thread safety stitch or add reinforcement.
Tunnelling and wavy hems are common on thin, light jerseys and show up more on cheaper fabrics. Inspection should include stretch testing a hem and a side seam by hand, and this is part of what an inspector checks against the standard in our quality control and AQL guide.
Fabric and Stretch Considerations
The fabric under the stitch changes everything. Lightweight single jersey needs a fine needle, moderate stitch density and gentle feed. Heavyweight French terry and fleece need a stronger needle and more allowance for loop pile, and the seam can be bulky. Fabrics with elastane need a stitch that stretches and recovers; if the fabric stretches more than the seam, the garment will pop. Our guide to stretch fabric covers how elastane affects seams.
A useful rule is to match seam stretch to fabric stretch. Stretch a swatch of the fabric across a ruler, then stretch a stitched seam on the same fabric; the seam should recover without distorting. Where a seam must hold under high tension, such as in the crotch of joggers, a five-thread safety stitch adds a chain stitch next to the overlock for extra strength.
Approving a seam on a first sample
- 1Pick the stitch by jobDecide whether the seam joins, hems or must lie flat, and choose overlock, coverstitch or flatlock to match.
- 2Match to the fabricCheck needle type, thread and stitch density against the actual fabric weight and stretch.
- 3Stretch test by handPull the seam and hem across their width and check they recover without popping or rippling.
- 4Wash and recheckWash the sample and look for puckering, skipped stitches and thread shrinkage.
- 5Record the settingsWrite the approved stitch type, density and thread into the tech pack for bulk.
A simple approval routine for knit seams; adapt it to your own quality procedure.
Following the same five steps on every new style keeps seam quality consistent from one season to the next.
How to Write Stitch Details in a Tech Pack
Stitch specification should be as plain as any other tech pack field. Name the seam, the stitch type, the number of threads, the density and the thread. If you have a reference garment, mark it up with notes, and photograph the seam inside and out. Our guide on how to make a tech pack shows where construction details sit.
Illustrative tech pack lines
- Shoulder and side seams: 4-thread overlock (ISO 514), 11 to 13 SPI, polyester core-spun thread, matched colour.
- Neck tape: coverstitch (twin-needle, as commonly 602), 8 to 10 SPI, stretch-matched.
- Hem and cuff attachment: overlock followed by coverstitch hem, or as per reference photo.
- Crotch and stress points: 5-thread safety stitch (ISO 516) or reinforced seam.
- Underwear or activewear seams: flatlock (as commonly 605 or 607), soft nylon loopers.
- Seam stretch requirement: seam must recover without popping on the hand stretch test.
- Needle: ballpoint or light ballpoint, size matched to fabric weight.
These are illustrative lines, not recommendations. Settings must be confirmed by the factory for your fabric and design.
Ask for a close-up photo of the first sample seams and a stretch test on each critical seam. If anything differs from your tech pack, record it, because stitch changes between samples and bulk are a frequent source of quality disputes.
How Ruhrose Approaches Seams and Stitches
Seam construction is part of every Ruhrose sample for hoodies, jersey and joggers, produced in Bangladesh with UK buyer support. We agree the stitch types, thread and density on the first sample, stretch-test the key seams and keep a record of the approved settings for bulk. For woven and denim garments the same discipline applies to lockstitch and chain stitch work. If your product depends on specialist flatlock construction for technical sportswear, we will say so early, because that sits outside our core range.