Flatlock Stitch: Everything You Need to Know

March 21, 2025

The flatlock stitch is a key technique in activewear, sportswear, and stretch fabrics, creating a flat, smooth, and flexible seam that prevents chafing and enhances durability. It’s commonly used in athletic and compression wear for a professional, comfortable finish.

In this guide, we’ll cover what a flatlock stitch is, how it differs from an overlock stitch, its types, and how to sew one. Whether you're a sewing enthusiast or a professional, mastering this stitch can take your garments to the next level!

What is a Flatlock Stitch?

A flatlock stitch is a type of seam that joins two pieces of fabric with no overlap, creating a flat and flexible finish. It is commonly used in activewear, sportswear, and stretch fabrics because it reduces bulk and prevents chafing. This stitch is created using a flatlock machine or a serger with specific settings. The process involves trimming the fabric edges, folding them slightly, and interlocking the threads to hold them securely. The result is a smooth, stretchy, and durable seam that looks great on both sides.

Why Does TBô Use Flatlock Stitching in Our Underwear?

Flatlock stitching is used in TBo bamboo underwear to provide maximum comfort and durability.

Since bamboo fabric is soft and breathable, the flatlock stitch complements it perfectly by maintaining a sleek, non-bulky finish that enhances the overall feel of the underwear. Additionally, flatlock seams are strong and flexible, ensuring that the underwear retains its shape and withstands regular wear and washing.

This thoughtful construction supports TBo’s commitment to comfort, performance, and sustainability.

Why Use a Flatlock Stitch?

A flatlock stitch is essential for garments that require comfort, flexibility, and durability. Whether you’re designing activewear, swimwear, or stylish sportswear, this stitch ensures a smooth, professional finish. It not only improves comfort but also enhances the overall appearance of your garments.

If you're working with stretchy fabrics and need seams that won’t irritate the skin, the flatlock stitch is the perfect choice!

Flatlock Stitch vs Overlock:

Many people confuse a flatlock stitch with an overlock stitch, but they serve different purposes. Here’s a clear comparison:

Flatlock Stitch:

Creates a flat seam with no fabric layers overlapping.
Ideal for sportswear, yoga pants, and swimwear because it reduces bulk and prevents chafing.
Uses a specific sewing technique where the fabric edges are slightly stretched and stitched together.
Produces a decorative effect on both the inside and outside of the garment.

Overlock Stitch:

Joins fabric edges with an overlapping seam and trims excess fabric.
Commonly used for finishing raw edges to prevent fraying.
Works well for sewing woven and knit fabrics but creates a seam that is not as flat as a flatlock stitch.
Typically used in casualwear, home textiles, and general garment construction rather than performance wear.

In short, if you need a flat, smooth seam for stretchy fabrics, a flatlock stitch is the better choice. If you’re just finishing fabric edges, an overlock stitch is more practical.

Flatlock Stitch Types:

There are different types of flatlock stitches, each with unique characteristics. Here are the most common ones:

1. Standard Flatlock Stitch

This is the most commonly used flatlock stitch. It creates a clean, flat seam with visible stitching on both sides. It’s perfect for athletic wear and compression garments, providing flexibility and durability.

2. Flat Seam with Decorative Coverstitch

This type combines a flatlock seam with a decorative coverstitch. It’s used when the seam is visible and meant to be part of the garment’s design. You’ll often see this in leggings, sports tops, and high-end activewear.

3. Reverse Flatlock Stitch

A reverse flatlock stitch places the looper threads on the outside, creating a raised, decorative effect. This is often used for fashionable sportswear or stylish outdoor apparel, giving the garment a bold, textured look.

4. Zigzag Flatlock Stitch

This version adds a zigzag pattern to the flatlock seam, increasing stretchability. It’s great for swimwear, cycling shorts, and other garments that require maximum flexibility.

5. Twin Needle Flatlock Stitch

Using a twin needle setup, this stitch offers a stronger seam with more elasticity. It’s often used in compression wear and high-performance sportswear to provide extra support.

What are the Benefits of a Flatlock Seam?

A flatlock seam offers several advantages, making it a preferred choice for many types of garments. Here are its key benefits:

1. Flat and Smooth Finish

Unlike traditional seams that create bulk, a flatlock stitch lies completely flat against the fabric. This reduces irritation and chafing, making it perfect for sportswear and athletic clothing.

2. Stretchable and Flexible

Since flatlock stitches work well with stretch fabrics, they allow garments to move with the body. This is essential for yoga wear, gym clothes, and swimwear, where flexibility is crucial.

3. Strong and Durable

Flatlock seams provide excellent durability, ensuring that garments withstand repeated washing and movement. This makes them ideal for high-performance clothing and compression wear.

4. Aesthetic Appeal

A flatlock stitch creates a clean, professional look. Since it is often visible on both sides of the fabric, it can serve as a decorative design element in modern sportswear and casual wear.

5. No Need for Extra Seam Allowance

Unlike traditional seams, flatlock stitching doesn’t require a seam allowance, making it an efficient and fabric-saving technique.

6. Reduces Chafing and Skin Irritation

Because the seam is flat, it prevents discomfort caused by raised or bulky seams rubbing against the skin, making it perfect for active and everyday wear.

How to Flatlock Stitch:

Creating a flatlock stitch may seem tricky at first, but with the right technique and settings, it becomes much easier. Follow this step-by-step guide to achieve a perfect flatlock seam.

Step 1: Prepare Your Fabric

Choose a stretch or knit fabric for the best results.
Cut your fabric pieces accurately, as the flatlock stitch creates an exposed seam that will be visible.

Step 2: Set Up Your Sewing Machine

If using a serger (overlock machine): Set it to flatlock mode by adjusting the thread tension and disabling the knife (so it doesn’t trim the fabric).
If using a regular sewing machine: Use a wide zigzag stitch with a loose tension to mimic a flatlock effect.

Step 3: Align the Fabric Edges

Place the two fabric pieces edge to edge, with a slight overlap (if using a regular sewing machine).
For a true flatlock stitch, the edges should meet without overlapping.

Step 4: Sew the Seam

If using a serger, stitch along the raw edges, allowing the threads to loop over and interlock the fabric.
If using a regular sewing machine, sew a wide zigzag stitch along the edge, ensuring the stitches catch both fabric edges evenly.

Step 5: Pull the Fabric Open

Once stitched, gently pull the fabric apart to flatten the seam. This exposes the loops and creates the signature flatlock effect.

Step 6: Press the Seam for a Clean Finish

Use a steam iron to press the seam open, making it lie completely flat.
This will enhance the stretch and comfort of the final garment.

Flatlock Stitch Sewing Machine:

A dedicated flatlock sewing machine is typically an industrial coverstitch or serger machine that can be adjusted for flatlock stitching. However, many home sewers achieve similar results using a serger or a regular sewing machine with specific settings.

1. Using a Serger (Overlock Machine) for Flatlock Stitching

A serger is the best option for creating a true flatlock stitch. Follow these steps:

Use 2 or 3 threads instead of 4 (to achieve a flatter seam).
Disable the knife blade, so it doesn’t trim the fabric.
Loosen the needle tension to allow the stitches to stretch and lie flat.
Increase the looper thread tension to create the signature flatlock effect.

2. Using a Regular Sewing Machine for Flatlock Stitching

If you don’t have a serger, you can still create a flatlock look with a standard sewing machine:

Use a zigzag stitch (set the width to medium and the length to short).
Sew along the fabric edges with a loose tension to allow the fabric to flatten.
Manually pull the seam open after stitching to create a similar effect.

Best Sewing Machines for Flatlock Stitching:

If you want to invest in a machine that can do flatlock stitching, consider these options:

Juki MO-735 – A high-quality serger that supports flatlock stitching with professional results.
Brother 1034D Serger – A beginner-friendly overlock machine that can be adjusted for flatlock stitches.
Janome CoverPro 1000CPX – A coverstitch machine ideal for flat seams and stretch fabrics.
Singer Heavy Duty 4432 – A regular sewing machine that can mimic a flatlock stitch using a zigzag stitch.

A flatlock stitch is a game-changer in the world of garment construction, especially for activewear, performance clothing, and everyday essentials. Its smooth, flexible, and durable finish ensures maximum comfort, making it a go-to choice for high-quality apparel. Whether you're looking for chafe-free seams, superior stretch, or long-lasting durability, mastering the flatlock stitch can elevate your sewing projects.

And if you're looking for underwear that benefits from the advantages of flatlock stitching, check out TBô Men’s Bamboo Underwear. Our underwear is designed for ultimate comfort, featuring:

Moisture-wicking fabric to keep you cool and dry all day.
Sustainable bamboo material, making it an eco-friendly choice.
Breathable and antibacterial properties for all-day freshness.
A man-shaped pouch, engineered for the perfect fit and support.

Experience the difference with TBô’s bamboo underwear, crafted with performance, sustainability, and comfort in mind. Shop now and upgrade your essentials with the best in men’s underwear!