Use the 4-thread overlock for any seam that has to hold and stretch: side seams, shoulders and necklines on knits, crotch seams, anything you will put weight on. Use the 3-thread overlock to finish raw edges before a conventional seam on wovens, for rolled hems and flatlock, and for seams in lightweight fabrics where the 4-thread stitch would show through as a ridge. The whole difference is the second needle: a 4-thread stitch has two needles and two loopers, a 3-thread stitch has one needle and the same two loopers.
| Feature | 4-thread overlock | 3-thread overlock |
|---|---|---|
| Needles | Two (left and right) | One (left for wide, right for narrow) |
| Loopers | Upper and lower | Upper and lower |
| Stitch width | About 6 mm (1/4 in), machine dependent | About 5 to 6 mm wide, 3 to 4 mm narrow (1/8 to 1/4 in) |
| How the seam is held | Two lines of needle thread, the inner one is the seam line | One line of needle thread, which is also the seam line |
| Stretch | High | High |
| Bulk | More | Less |
| Thread used | Most | About a quarter less |
| Best for | Seams in knits, durable seams in casual wovens | Edge finishing, rolled hems, flatlock, light fabrics |
What is the difference between a 4-thread and a 3-thread overlock?
In any overlock stitch, the two loopers interlace their threads over the cut edge: the upper looper lays thread across the top, the lower looper across the underside, and they meet at the edge. The looper threads do not go through the fabric. What anchors the stitch to the fabric is the needle thread, which pierces the cloth and catches the lower looper thread on each stitch.
A 3-thread overlock uses one needle. Put it in the left position and you get the wide version, which finishes a 5 to 6 mm (about 1/4 in) edge; put it in the right position and you get the narrow version at 3 to 4 mm (1/8 to 5/32 in), the base for rolled hems.
A 4-thread overlock uses both needles at once. The left needle sews the seam line; the right needle adds a second line of stitching between the seam and the edge. The loopers wrap over the whole width as before. Because that second line backs up the first, the stitch is sometimes called a mock safety stitch, and it is the reason a 4-thread seam survives a popped thread where a 3-thread seam opens.
Most domestic sergers sold at the time of writing (October 2026) sew both, plus a 2-thread finish with a converter on the upper looper. Older and cheaper machines may be 3-thread only, which is enough for finishing but a compromise for knit seams.
When should you use a 4-thread overlock?
For seams. On jersey, French terry, rib and interlock, the 4-thread stitch sews, trims and finishes the seam in one pass, and it stretches as far as the fabric does without the thread snapping. A T-shirt, a sweatshirt or leggings can be sewn almost entirely with it, and sewing knits on a regular machine explains what to do for the hems and necklines it cannot handle.
On wovens, use it where speed and durability matter more than a flat, pressed-open seam: pajamas, children's clothes, tote bag linings, the inside of a bucket hat, the long seams of a gathered skirt. For a fitted woven dress or trousers, sew the seam on the sewing machine first and finish the allowances with a 3-thread, so you can press the seam open and let it out later.
Mind the seam allowance. A 4-thread seam takes up roughly 6 mm from the needle to the edge, so with a 1.5 cm (5/8 in) allowance you run the knife to trim 9 mm (3/8 in) away and the left needle lands on the seam line. With a 1 cm (3/8 in) allowance, which is what Sewly draws on knit patterns by default, you trim about 4 mm (5/32 in). Either way, sew a test seam on scraps and measure the finished width before you touch the garment.
When is a 3-thread overlock the better choice?
For edges and for anything light. Finishing the allowances of a woven seam with a wide 3-thread, before or after sewing it conventionally, gives a flat edge that presses open cleanly. The 4-thread would do the same job with more bulk and more thread for no gain.
The narrow 3-thread is the setting for sheer and fine fabrics: chiffon, lawn, georgette, silk crepe. A 6 mm wide stitch on a lawn blouse leaves a stiff ridge you can feel and see from the outside; the narrow stitch, or a 2-thread finish if your machine has it, lies flat.
It is also the base of the two stitches that are not seams at all. The rolled hem uses the right needle with the stitch finger retracted, upper looper tension loosened and lower looper tightened so the edge curls under inside the stitch; how to sew a rolled hem covers the settings. Flatlock loosens the needle tension and tightens the lower looper so the seam pulls open flat, with ladders on one side and loops on the other; it is decorative and reversible but not strong.
Lastly, the 3-thread can sew seams in knits when bulk matters more than security: lingerie-weight jersey, a knit slip, the bands of a lightweight top. Those seams hold, but the needle thread is the only thing holding them.
Does a 3-thread seam hold up in clothes?
In low-stress places, yes. Under strain, at the crotch, the underarm, a tight waistband, a 3-thread seam can grin: the fabric pulls apart along the seam line and the looper threads show between the two layers. If the single needle thread breaks, the seam runs open from that point. The 4-thread's second line of stitching stops both.
As an edge finish, there is no difference between the two: a 4-thread used only to overcast is a 3-thread with an extra line of thread on the allowance, which you will never see.
How do you switch between the two stitches?
- Lower the needles and remove the one you do not need: the right needle for a wide 3-thread, the left for a narrow one. Pull its thread out of the way or unthread it from the guides so it cannot be caught.
- Return the tensions to the machine's normal values and set the stitch length to 2.5 to 3 mm (about 1/8 in) for seams, shorter for rolled hems.
- Sew 10 cm (4 in) on two layers of your garment fabric, pull the seam across the stitching and look at both sides. Looper threads should meet exactly at the edge; adjust the upper looper if they meet underneath, the lower looper if they meet on top.
- Write the settings on a scrap and pin it to the machine. Each fabric gets its own.
If you serge a lot of both, a second serger is not the only answer: many people keep theirs set up for the 4-thread and finish fine wovens on the sewing machine, as the comparison of seam finishes shows.
Which should you choose?
Use the 4-thread when you are sewing a garment seam in any knit, a seam in a casual woven that will be washed hard, or a seam you want to trust without a second row of stitching. If you only ever learn one serger setting, make it this one.
Use the wide 3-thread when you are finishing the allowances of a woven seam that will be pressed open, or sewing seams in a light knit where a ridge would show.
Use the narrow 3-thread when the fabric is sheer or fine, for a rolled hem on a scarf, a ruffle or a circle skirt, or for flatlock as a visible detail.
Buying a serger? Make sure it sews both, has differential feed and a retractable stitch finger for the rolled hem. A 3-thread-only machine will finish edges beautifully and leave you sewing knit seams twice. If you are weighing a serger against the overlock stitches on your sewing machine, overlock stitch on a regular machine vs a serger gives the honest answer.