How a lockstitch is formed
A lockstitch is made by a needle thread and a bobbin thread that interlock inside the fabric. The needle carries the top thread through the material, a hook underneath catches it and pulls a loop around the bobbin case, and the two threads lock together in the middle of the seam. Because the threads cross at the centre, the stitch cannot unravel from a single broken loop the way a chain stitch can, and it does not need a raw edge to work.
The result is a flat, dense, two-thread seam. On the top face you see a straight row of stitches; on the underside you see the same row, with the bobbin thread showing. Tension balance decides which side the knot sits on, and a balanced seam hides the interlock inside the fabric layers.
Lockstitch behaviour in practice:
- Very little stretch along the seam, which makes it ideal for woven fabrics and for seams that must hold a shape.
- Strong in both directions and resistant to running, so it suits stress points such as armholes, waistbands and pocket mouths.
- Does not finish a raw edge by itself; the edge must be pressed open, bound, or overlocked separately.
- Works on virtually any fabric weight, from lightweight shirting to denim and outerwear.
A direct-drive lockstitch such as the AS500-1S6 is a typical example of the machine type used for this work. For a fuller picture of the category, see Lockstitch Sewing Machines.
How an overlock is formed
An overlock, often called a serger, uses two, three, four or five threads and a set of loopers rather than a bobbin. The needle threads pass through the fabric while upper and lower loopers carry threads around the raw edge, wrapping it as the fabric advances. The stitch both sews and edge-finished in one pass, which is why overlockers dominate side seams, hems and any operation where the cut edge must be contained.
Seam appearance differs from a lockstitch in an important way. On the face side, an overlock usually shows a row of parallel needle stitches close to the edge, with the looper threads wrapping over the cut edge. On the back, the loop structure is visible as a ladder of threads covering the raw edge. The seam is narrow and sits at the very edge of the fabric rather than down the middle of the join.
Overlock behaviour in practice:
- The loop structure gives the seam considerable stretch and recovery, which is why it is standard on knitwear and on woven garments that need movement.
- It trims the edge as it sews on machines with an integrated knife, producing a clean, uniform edge.
- It is not a load-bearing seam on its own in many constructions; overlocked seams are often reinforced by a lockstitch or a coverstitch elsewhere.
- Thread density and differential feed settings control whether the edge stretches, gathers or lies flat.
The category page at Overlock Sewing Machines covers the machine configurations available.
How an interlock (coverstitch) is formed
An interlock stitch, commonly delivered by a coverstitch machine, is a multi-thread stitch that forms loops on both faces of the fabric. Two or more needle threads run parallel on the top face, while the bottom face is covered by a dense network of looper threads. Because the loops sit on both sides, the seam looks finished from either direction and the reverse side is not a raw ladder of threads.
This double-faced coverage is what separates interlock from overlock. An overlock wraps an edge; an interlock covers a seam area and, in the process, can hold a folded hem or a joined panel with stretch.
Interlock behaviour in practice:
- High stretch and excellent recovery, making it the standard finish for knit hems, necklines, cuffs and binding.
- The top face shows clean parallel needle lines; the bottom face shows a flat, covered surface.
- It is generally not used to join heavy woven seams that must hold a fixed shape.
- Folded hems finished with interlock keep their width and do not crack when the garment stretches.
See Interlock Sewing Machines (Coverstitch) for the machine family.
Side-by-side comparison
| Feature | Lockstitch | Overlock | Interlock (Coverstitch) |
|---|---|---|---|
| Threads | Two (needle and bobbin) | Two to five, loopers instead of bobbin | Multiple needle and looper threads |
| Stitch formation | Threads interlock inside the fabric | Loopers wrap the raw edge | Loops form on both faces |
| Face appearance | Straight row of stitches | Needle row near the edge | Parallel needle lines |
| Back appearance | Matching stitch row | Looper ladder over the edge | Flat covered surface |
| Stretch | Minimal | High | High, with strong recovery |
| Edge handling | None; edge finished separately | Trims and wraps the edge | Covers a folded or joined edge |
| Typical use | Structural seams, wovens | Side seams, edge finishing, knits | Knit hems, necklines, cuffs |
Typical garments and operations for each
Lockstitch is the backbone of woven construction: shirt side seams and yokes, trouser inseams, jacket panel joins, pocket bags, waistband attachment, zipper setting and most topstitching. Anywhere a seam must hold a precise shape and not stretch out of line, a lockstitch is the default.
Overlock is the default for edge containment: side seams and armhole seams on knit tops, the raw edges of woven panels before assembly, hem allowances, and the seams of garments that must move with the body. It is also common as a preparatory step, overlocking pattern pieces before they are joined by lockstitch.
Interlock is the finishing stitch of knitwear: T-shirt and sweatshirt hems, ribbed cuffs, neckline binding, and the leg openings of knit bottoms. It is chosen where the finished garment will be seen from both sides and where the hem must stretch without cracking.
Why a factory line needs all three
A production line that makes a single knit T-shirt may use an overlock for the body seams and an interlock for the hems, with a lockstitch for the neck binding join or a label attachment. A woven shirt line uses lockstitch throughout the body and overlock for edge cleaning. Very few garments are made with one stitch type alone.
The reason is functional, not traditional. Each stitch solves a different problem: holding a shape, containing an edge, or covering a stretch area. A line that tries to substitute one for another usually pays for it in repair rates, seam failures or a hem that will not lie flat. Planning the stitch sequence per operation, rather than per garment, is how factories keep both quality and throughput stable.
Common beginner mistakes
The most frequent error is using an overlock where a lockstitch seam is required. An overlock seam is narrow and stretchy; on a woven garment that needs a strong, stable join, it can pull open under load. Structural seams belong to the lockstitch.
Other mistakes worth knowing:
- Overlocking a seam and treating it as finished when the seam still needs a lockstitch for strength.
- Using a lockstitch on knit hems, which produces a seam that cracks when stretched.
- Skipping the interlock on knit necklines, leaving a hem that rolls or loses shape after washing.
- Ignoring differential feed on the overlock, causing wavy or gathered edges on stretch fabrics.
- Mismatching thread and needle size to the fabric, which shows up as skipped stitches or broken loops.
- Assuming a coverstitch and an overlock are interchangeable because both have loopers; the seam structures are different.
Buyer checklist before choosing stitch types
- Confirm the fabric: woven or knit, weight, and expected stretch at the seam.
- List each operation and decide whether it needs shape retention, edge containment or stretch coverage.
- Decide which seams will be seen from both faces; those are interlock candidates.
- Check whether the overlock will trim as it sews, or whether edges are pre-cut.
- Plan thread types and needle sizes per operation before sampling.
- Confirm machine specifications, voltage and configuration on quotation, since these vary by order.
For a quotation covering lockstitch, overlock or interlock machines, or a mixed line configuration, please request a quotation via the contact page and include your fabric types, operations and target output so the right configuration can be confirmed. Certificates available upon request.