The diagonal seam method joins two bias strips into one long piece

To sew bias tape strips together, lay two strips right sides together at a 90-degree angle, forming an L shape. Draw a diagonal line from the top-left corner to the bottom-right corner of the overlap, then sew along that line. Trim the excess fabric to a quarter-inch seam allowance and press the seam open. This diagonal join is nearly invisible when the tape is folded and applied to fabric, because the seam runs along the bias grain rather than straight across.

The diagonal method works because bias tape is cut at a 45-degree angle to the fabric grain. When you join two strips at right angles and sew diagonally, the seam aligns with that grain, so it sits flat and doesn't create a thick ridge when you fold the tape in half. A straight seam across the width of the tape would bunch and show.

Key Takeaways

  • Lay the two strips right sides together in an L shape, with one strip rotated 90 degrees, and sew a diagonal line from corner to corner.
  • Trim the seam allowance to a quarter inch and press it open before using the joined tape.
  • The diagonal seam lies flat when the tape is folded because it runs along the bias grain of the fabric.
  • A straight seam across the width would create bulk and show as a ridge when the tape is applied to a garment or project.

Setting up the strips correctly before sewing

Cut two bias strips to the width you need — typically 1.25 inches for single-fold tape or 2.25 inches for double-fold. Place them on a flat surface with right sides facing up. Pick up one strip and rotate it 90 degrees, then place it on top of the other so the corners overlap in an L. The overlap should be roughly the width of one strip — about an inch or so.

Pin the overlap in place to keep the strips from shifting while you mark the diagonal line. Use a ruler and a fabric pencil or chalk to draw a line from the top-left corner of the overlap to the bottom-right corner. This line should run at a true 45-degree angle. If you have a rotary cutter and a 45-degree angle ruler, use those to mark the line — they are more accurate than a pencil.

Sewing the diagonal seam

Thread your machine with thread that matches the bias tape or the project you are working on. Set the stitch length to 2.0 or 2.5 — a standard length that will hold the seam without puckering. Sew directly along the diagonal line you marked, starting at one corner and ending at the other. Backstitch at both ends to lock the seam.

Sew slowly and keep the presser foot aligned with the marked line. If the line is hard to see under the presser foot, mark it with a pen on the presser foot itself — a small dot at the quarter-inch mark — so you can follow it visually. Remove the pins as you approach them, not before you start sewing.

Trimming and pressing the seam

After sewing, trim the excess fabric from the overlap. Leave a quarter-inch seam allowance on both sides of the stitching line. Use a rotary cutter and ruler or sharp scissors to cut away the triangular flaps of fabric that stick out beyond the seam allowance. Be careful not to cut into the seam itself.

Press the seam open with a hot iron. Lay the joined strips flat on an ironing board and press from the back side first, then flip and press from the front. The seam should lie flat and the two strips should form one long, straight piece. If the seam puckers or the strips are not aligned, you may have trimmed too close or sewn off the marked line — unpick the seam and try again.

Checking the join before you use it

Fold the joined tape in half lengthwise, the way you would fold it for process to a garment. Run your finger along the seam from both sides. You should feel almost no ridge or bump. If the seam creates a noticeable lump, the diagonal angle was off or the seam allowance is too wide. Trim it down further if needed.

Lay the folded tape flat on a pressing surface and press it again to set the fold. The seam should disappear into the fold. If the tape twists or the seam pulls to one side, unfold it and check that the seam is truly centered. A centered seam will fold evenly on both sides.

Joining more than two strips

If you need a very long piece of tape, join the first two strips using the diagonal method, then join the result to a third strip the same way. Sew the diagonal seam, trim, and press. Repeat for as many strips as you need. Each join adds only a small amount of bulk, and the diagonal seams stack neatly if you press each one open before adding the next strip.

Some sewers prefer to chain-piece the strips — sewing one diagonal seam, then when ready sewing the next without cutting the thread between them. This saves thread and time, but you will still need to trim and press each seam individually. The order does not matter as long as each seam is pressed open before you fold the tape for use.

Why the diagonal seam matters for finished projects

A diagonal seam is nearly invisible on a finished garment or project because it runs along the bias grain and sits flat when the tape is folded. A straight seam across the width of the tape would create a thick ridge that shows through on the right side of the fabric, especially on lightweight materials like cotton voile or linen. The diagonal method is the standard in garment sewing and quilting for this reason.

If you are binding a quilt or hemming a neckline, the seam will be hidden inside the fold of the tape, so it will not be visible from either side of the finished project. This is why the diagonal method is worth the extra step — it ensures a professional finish that will not show seams or bulk.

Frequently Asked Questions

Can I use a straight seam instead of a diagonal one?

You can, but the seam will create a ridge when the tape is folded. A straight seam across the width of the tape doubles the fabric thickness at that point, and it will show on the finished project. The diagonal seam distributes the thickness along the bias grain, so it lies flat.

What if my strips are different widths?

Trim both strips to the same width before joining them. If one is slightly narrower, trim the wider one to match. Uneven widths will cause the joined tape to twist or fold unevenly when you explore it to your project.

Do I need to mark the diagonal line with a pencil?

Marking is not required if you are confident sewing at a 45-degree angle by eye, but it takes only a minute and ensures accuracy. A marked line is especially helpful if you are new to sewing bias tape or if your machine does not have a 45-degree angle guide on the presser foot.

How much seam allowance should I leave?

A quarter inch is standard and works well for most bias tape. If your tape is very narrow (under an inch wide), you may trim to an eighth inch to reduce bulk. Never leave more than a quarter inch, or the seam will be too thick to fold smoothly.

Can I press the seam to one side instead of open?

Pressing to one side is not recommended for bias tape because it creates more bulk on one side of the seam. Pressing open distributes the thickness evenly and allows the tape to fold flat on both sides.