Flatlock or overlock seams: which is better for underwear?

Short answer: Overlock seams join two layers and finish the cut edge in one operation, leaving a small raised seam allowance inside the garment. They are strong, stretchy and quick to sew. Flatlock seams bring the edges together flat and cover them with stitching on both faces, so there is very little ridge against the skin. Flatlock is usually more comfortable in high-friction areas, but it is more visible, needs suitable machines and careful setting, and often costs more. Many good boxer briefs mix both: flatlock where the skin rubs, overlock where it does not.

How each seam is built

An overlock (serger) seam uses needles and loopers to stitch and wrap the cut edge, producing a narrow seam allowance that sits on the inside. A flatlock seam either butts the two edges or overlaps them slightly, then covers them with a cover stitch so the seam lies flat. The stitches show on the outside as a decorative ladder or band, which gives a sporty look.

Trade-offs

• Comfort: flatlock is flatter and less likely to rub; overlock can leave a ridge that some customers feel.
• Strength and stretch: both can stretch well with suitable thread and stitch settings, but either fails if the settings do not match the fabric's extension.
• Appearance: overlock is hidden; flatlock is visible and becomes a design element.
• Cost and capacity: flatlock usually needs dedicated machines and more setting time, and not every line has them free.
• Fabric: flatlock works well on stable knits; very thin or slippery fabrics can pucker or ladder.

Where to use each

• Flatlock: inner thigh, crotch and side seams of sports, compression and long-leg boxer briefs.
• Overlock: low-friction seams, or styles where a clean exterior matters more.
• Mixed: a common approach is flatlock on leg and crotch seams and overlock on the pouch or waistband join.

Testing before switching

Make the same style with both seam types and wear-test them side by side while walking, sitting and exercising. Stretch the seam to the fabric's working extension and check for popped stitches. Wash several cycles using a recognised domestic procedure such as ISO 6330, then measure garment dimensions with a method such as ISO 5077 to confirm seams do not distort or shrink differently from the fabric.

Details that matter as much as seam type

Thread choice matters: soft textured threads in the loopers reduce roughness. Stitch density and tension affect both comfort and strength, and seam placement, moved away from the point of highest friction, can solve rubbing on its own.

Limits

A badly set flatlock can still rub, and a well-made overlock can be comfortable. Results depend on fabric, thread, fit and placement together.

Facts, recommendations and uncertainty

Established: flatlock seams are flatter and usually more comfortable against the skin; overlock seams are hidden, strong and economical. Recommendation: use flatlock in high-friction areas and confirm by wear and wash testing. Uncertain: whether the cost increase is justified for your price point and customers.

Sources and further reading

• ASTM D4964 — Tension and elongation of elastic fabrics
• ISO 6330:2021 — Domestic washing and drying procedures for textile testing
• ISO 5077:2007 — Determination of dimensional change in washing and drying

Standards and regulations are referenced by their published scope. Test parameters, cycles and pass levels should be agreed with your laboratory for the specific product; legal requirements should be confirmed for each destination market.