A twin needle is the usual answer for hemming knits on a regular sewing machine: two parallel rows on the right side, a zigzag of bobbin thread underneath, and enough stretch for T-shirts, dresses and most everyday knits. A coverstitch machine makes the same two-row hem with more stretch and a looper thread that covers the raw edge, but it is a second machine. A plain zigzag or a built-in stretch stitch is the quickest and the stretchiest, and it looks homemade on light fabrics. If you own one machine, start with a twin needle; buy a coverstitch when you hem knits every week; keep the zigzag for activewear, swimwear and anything where stretch matters more than looks.
How do the three knit hems compare?
| Feature | Twin needle | Coverstitch | Zigzag or stretch stitch |
|---|---|---|---|
| Machine | Any zigzag sewing machine with a second spool pin | A coverstitch machine, or a serger-coverstitch combo | Any sewing machine |
| Look on the right side | Two straight parallel rows | Two or three parallel rows, as on ready-to-wear | Visible zigzag |
| Stretch | Moderate | High | High, more with a wider stitch |
| Raw edge | Left raw or finished first | Covered by the looper thread on the inside | Left raw or enclosed in the fold |
| Typical trouble | Tunneling, skipped stitches, popped threads | Threading, unpicking, cost | Wavy hem, stitches that show |
| Extra cost | One needle | A machine | Nothing |
How do you hem a knit with a twin needle?
A twin needle is two needles on one shank. It sews two straight rows on top while the single bobbin thread zigzags between them underneath, and that zigzag is what lets the hem stretch. Most machines that do a zigzag accept one; you need a second spool pin (or a bobbin wound with thread and set on the pin as a second spool) and the needle plate must have a wide enough hole.
Choose a stretch twin needle, which has a rounded point, in a 4 mm (5/32 in) width for most hems; 2.5 mm looks fine on light jersey and 6 mm suits sweatshirt fleece. Thread the two spools so they unwind in opposite directions, which reduces tangling, and run both threads through the usual path, one through each side of the tension disc if your manual says so.
Then press the hem allowance up and stitch from the right side, so the two rows land where you want them and the zigzag catches the raw edge below. Lengthen the stitch to 3 to 3.5 mm, lower the top tension a notch and sew at an even speed. The knit fabric guide covers the rest of knit sewing on an ordinary machine.
When does a coverstitch machine earn its place?
A coverstitch machine uses two or three needles and one looper. The looper thread forms a flat chain on the underside that covers the raw edge, which is why a store-bought T-shirt hem shows two straight rows outside and a ladder of thread inside. The stitch stretches far more than a twin-needle hem and does not pop when you pull the garment over your head.
It also handles necklines, binding, and the flat seams of activewear. What it does not do is sew a seam or cut an edge: it is a third machine next to the sewing machine and the serger, with its own threading and a stitch that unpicks easily from the wrong end and stubbornly from the right one. Coverstitch vs serger goes through what each one does if the two are blurring in your mind.
The honest test is frequency. If you hem a knit every month, a twin needle is enough. If you sew a drawer of T-shirts and leggings each season, the coverstitch pays back in time and in hems that no longer need a second attempt.
When is a zigzag hem the right choice?
A zigzag has one advantage no other hem matches on a single machine: it stretches as far as the fabric does. A narrow zigzag, about 1 mm wide and 3 mm long, looks almost like a straight line from a distance and suits leggings, swimwear and dance wear, where a hem that pops is worse than a hem that shows. Most machines also offer a stretch stitch (often a lightning-bolt symbol) and a three-step zigzag, both good on elastic fabrics.
Its weakness is the look. On a drapey viscose jersey the zigzag shows, and the fabric between the points can pucker. It is still the right choice for a child's T-shirt, a hem you expect to redo, or a fabric too thick for a twin needle.
Why do twin-needle hems tunnel, and how do you stop it?
Tunneling is the ridge that forms between the two rows: the bobbin thread pulls the two lines toward each other and the fabric rises. Three things fix most of it.
- Lower the top tension. Try one or two numbers below normal. The zigzag underneath loosens and the fabric lies flat.
- Stabilize the hem. A strip of fusible knit interfacing or wash-away stabilizer in the hem allowance gives the needles something firm to pierce. Some sewists use a 1 cm (3/8 in) strip of fusible hemming tape, which also holds the fold in place while you sew.
- Use a narrower needle. A 2.5 mm twin needle tunnels less than a 4 mm one, because there is less fabric between the rows to pull up.
Skipped stitches come from the wrong needle (a universal point instead of a stretch or ballpoint), a needle that is bent, or sewing too fast over a bulky side seam. Slow down at the seam and walk the needle over the thickest part by hand.
How do you keep a knit hem from going wavy?
Waviness has the same cause with all three methods: the fabric stretches as it goes under the foot, and the stitches lock the stretch in. Let the fabric feed itself without pulling, lower the presser foot pressure if your machine allows it, and use a walking foot on a regular machine, which feeds the top layer at the same rate as the bottom. Stabilizing the hem helps here too. A final press with steam, the iron lowered rather than slid, shrinks a slight wave back out of most cotton jerseys.
Press the hem up before you stitch rather than folding as you go. On a 1 cm (3/8 in) seam allowance, which is what Sewly's knit patterns include by default, a 2 to 2.5 cm (3/4 to 1 in) hem is a comfortable depth to fold and catch with the needles. The hemming guide shows how to mark and press a hem evenly.
Which should you choose?
- You own one sewing machine and sew a few knits a year: a stretch twin needle in 4 mm width, stabilizer in the hem, lower top tension. It covers T-shirts, sweatshirts and knit dresses.
- You sew knits weekly, or you make activewear, children's clothes or garments to sell: a coverstitch machine. The stretch and the finish are worth the threading.
- Leggings, swimwear, dance costumes, or a fabric with more than 50 % stretch: a narrow zigzag or the stretch stitch. The hem must stretch with the fabric, and the look matters less where it sits.
- A thick or spongy knit such as fleece or scuba: zigzag or a 6 mm twin needle; a coverstitch also handles it well.
- A sheer or very light jersey: twin needle at 2.5 mm with wash-away stabilizer, or a coverstitch if you have one.
Whichever you pick, test on a scrap of the same fabric folded the same way, then stretch the sample hard. If threads pop, lengthen the stitch, loosen the tension or switch to a stretchier method before you hem the garment. A T-shirt is the right first project to practice on: the hems are straight and short, and a second try costs nothing.