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Sewing patterns for knit garments love to say things like “finish the seam with an overlock stitch” or “hem with a coverstitch” as if everyone owns three machines and knows the difference. Most sewists own one machine, maybe a serger if they are lucky, and these three terms blur together because all three involve loopers, all three show up on activewear, and all three get called “serging” by somebody.
They are different stitches, made by different machines, for different jobs. An overlock wraps a raw edge. A flatlock joins two edges flat without overlap. A coverstitch hems without trimming. Mix them up and you get the wrong tool for the job, like trying to hem a t-shirt with a seam finish.
This guide explains what each stitch is, what it looks like from both sides, which machine makes it, and when to reach for each one.
TL;DR: An overlock stitch wraps thread around a raw edge to finish and seam in one pass, and it is made by a serger. A flatlock stitch butts two fabric edges together with bridging looper threads so the seam lies flat, and it is made by a serger set to flatlock or a coverstitch machine. A coverstitch shows two or three parallel lines on top with looper coverage underneath, and it is made by a coverstitch machine for hemming knits. Use overlock for seams and edge finishing, flatlock for flat decorative seams on activewear, and coverstitch for professional knit hems.
The Overlock Stitch: Edge Finishing and Seaming
What it looks like: Imagine the raw edge of fabric wrapped in a neat blanket of thread. From the top you see the needle thread as a straight line just inside the edge, with looper threads wrapping around the edge itself. From the underside you see the interlocking looper threads forming a tidy chain along the edge. The fabric edge is trimmed clean by the serger’s knife as it sews, so the finished edge is perfectly even.
Which machine makes it: A serger (overlock machine). The standard 4-thread overlock uses two needles and two loopers. A 3-thread overlock uses one needle and two loopers and is the classic edge finish for single layers. Most home sergers do both.
When to use it: This is the workhorse. Use an overlock to seam knit garments (t-shirts, leggings, sweatshirts) because the stitch stretches with the fabric. Use it to finish raw edges on woven garments so they do not fray in the wash. Use it to sew and finish in a single pass, which is why garment factories love it. If you only learn one serger stitch, this is the one.
What it cannot do: It trims as it sews, so it is wrong for hems where you need to keep the full fabric width. And while the edge looks tidy, the top of the stitch is not decorative in the way a coverstitch hem is.
The Flatlock Stitch: Flat Seams, Decorative and Functional
What it looks like: Picture two pieces of fabric laid edge to edge, not overlapping, joined by threads that bridge the gap like the rungs of a tiny ladder. From one side you see straight parallel needle lines with the bridging threads visible between them; from the other side you see the looper threads forming loops across the join. Pulled open flat, the seam has almost no bulk, which is the entire point.
Which machine makes it: A serger adjusted for flatlocking (tension settings changed so the seam pulls open flat instead of gathering the edge), or a coverstitch machine, which does flatlock-style seaming natively. Some serger manuals include a flatlock setup; others do not, so check yours.
When to use it: Activewear and swimwear, where bulky overlapping seams would chafe against skin. Decorative exposed seams on sweatshirts and knit tops, where the ladder effect becomes a design feature, often sewn with contrasting thread. Anytime you need two layers joined with minimum bulk, like piecing quilt batting or joining fleece.
Flatlock vs. overlock in practice: An overlock overlaps the seam allowances and wraps the edge. A flatlock butts the edges and bridges them. Overlock is stronger for structural seams; flatlock is flatter and more comfortable against skin. On a store-bought rash guard, the seams that feel like nothing against your skin are flatlocked.
The Coverstitch: Professional Knit Hems
What it looks like: This is the stitch on the hem of every ready-to-wear t-shirt. From the top (the outside of the garment), you see two or three perfectly parallel lines of straight stitching. From the underside, a looper thread weaves back and forth covering all the needle threads, which is why it is called a coverstitch: the looper covers the underside. Nothing is trimmed. The full fabric width stays intact.
Which machine makes it: A coverstitch machine, a dedicated machine with two or three needles and one looper and no knife. Some serger-coverstitch combination machines exist, but a standalone coverstitch machine is the standard home solution. A twin needle on a regular sewing machine imitates the top appearance, with a zigzag underneath instead of true looper coverage.
When to use it: Hemming knit garments: t-shirts, leggings, dresses, cuffs, and neckline bindings. Anytime you want a hem that stretches with the fabric and looks store-bought from the outside. It is also used for decorative topstitching and for attaching elastic and bindings on activewear.
Coverstitch vs. overlock in practice: They are complements, not competitors. The overlock constructs the garment’s seams and finishes edges; the coverstitch finishes the hems. A typical t-shirt is sewn with overlock seams at the shoulders and sides, then hemmed with a coverstitch. One machine cannot do the other’s job because the overlock trims and the coverstitch does not.
Side-by-Side Comparison
Think of it this way:
- Overlock trims the edge and wraps it in thread. Seams plus edge finishing. Made by a serger.
- Flatlock joins edges without overlap so the seam lies flat. Comfort seams and decorative exposed seams. Made by a serger in flatlock setup or a coverstitch machine.
- Coverstitch sews parallel top lines with underside looper coverage and trims nothing. Knit hems. Made by a coverstitch machine.
If your pattern says “finish the seam,” that is overlock. If it says “hem the knit top,” that is coverstitch. If it shows exposed seams on activewear that lie perfectly flat, that is flatlock.
Which Machine Do You Actually Need?
Be honest about what you sew. If you mostly sew woven garments and want tidy insides, a serger for overlocking covers you completely. If you sew a lot of knits, a serger handles the seams and a twin needle on your regular machine can fake the hems convincingly for a long time.
The coverstitch machine earns its place when you sew knits constantly and want true ready-to-wear hems, or when twin-needle hems keep tunneling and frustrating you. It is a luxury, not a necessity, but owners report it is the machine they did not know they needed until they tried one.
For machine guidance, these deep dives cover the options: the best sergers for beginners if overlocking is your gap, and the best coverstitch machines if knit hems are your frustration. The Juki MCS-1500 vs Brother 2340CV comparison settles the most common coverstitch buying question.
Common Mistakes to Avoid
Using an overlock stitch to hem. The knife trims your hem allowance away and the edge finish looks wrong on a folded hem. Overlock is for seams and raw edges; hems want a coverstitch or twin needle.
Expecting a serger to coverstitch. Standard sergers cannot make a true coverstitch because they have knives and different looper geometry. Combination machines exist but involve tedious changeovers. They are different tools.
Flatlocking without adjusting tension. A serger’s default tensions produce a gathered overlock edge, not a flat seam. Flatlocking requires specific tension changes (usually loosening the needle tension dramatically) so the seam pulls open flat. Follow your manual’s flatlock settings exactly.
Confusing the terms when buying patterns or machines. If a pattern calls for a coverstitch hem and you only own a serger, plan for a twin-needle hem instead. Knowing which stitch is which saves you from buying the wrong machine for the job.
FAQs
Can a serger do a coverstitch?
A standard serger cannot. The stitches use different looper configurations, and the serger’s knife is in the way. Combination serger-coverstitch machines exist, and a twin needle on a regular machine imitates the coverstitch look on top.
What is the difference between flatlock and overlock?
An overlock overlaps seam allowances and wraps the raw edge in thread, trimming as it goes. A flatlock butts two edges together without overlap and bridges them with looper threads, producing a seam with almost no bulk.
Do I need a coverstitch machine for sewing knits?
No. A serger handles knit seams, and a twin needle on a regular machine produces convincing knit hems. A coverstitch machine is worth it when you sew knits heavily and want true ready-to-wear hems without fuss.
Which stitch is on the hem of my t-shirt?
A coverstitch: two parallel lines on the outside, looper coverage on the inside. The side seams and shoulder seams are overlocked.
Can I flatlock on any serger?
Most sergers can be set up for flatlocking with tension adjustments, but not all manuals document it. Check your manual for flatlock settings. Coverstitch machines also sew flatlock-style seams.
Why do my overlocked edges wave or pucker?
Usually a tension or differential feed issue. Loosen the looper tensions slightly for waving, and adjust the differential feed: increase it above 1.0 for puckering on lightweight fabric, decrease it below 1.0 for stretching on knits.