The diagonal seam method joins two strips without bulk

The fastest way to join bias binding strips is to lay them perpendicular to each other, mark a diagonal line, and sew along it. This creates a seam that runs at 45 degrees to the length of the strips — the same angle as the bias itself — so the seam sits flat when you fold the binding and press it. The waste triangle gets trimmed away, leaving you with one long continuous strip.

This method works because bias fabric naturally stretches along the diagonal. When your seam runs at that same angle, the binding can flex without puckering or pulling. A straight seam across the width would fight the grain and create a stiff, visible line.

Key Takeaways

  • Place two strips right sides together at a 90-degree angle, with the corners overlapping slightly, and mark a diagonal line from corner to corner before sewing.
  • Sew along the marked line, then trim the seam allowance to 1/4 inch and press the seam open to reduce bulk in the finished binding.
  • Test the seam by folding the joined strip in half lengthwise — it should lie flat without puckering or twisting.
  • Chain multiple strips together in one session to save time and thread, then press all seams before you begin binding your quilt or garment.

Setting up the strips at the right angle

Lay your first strip on the table with the right side up. Take your second strip and place it on top, right side down, so the two strips form an L shape. The corner of one strip should overlap the corner of the other by about 1/2 inch. You are aiming for the strips to cross each other, not sit end to end.

Use a ruler and a pencil or fabric pen to draw a diagonal line from the upper left corner of the top strip to the lower right corner. This line should run at 45 degrees — the same angle as the bias grain. If you have a 45-degree ruler, align it with the corners and draw along the edge. If not, a regular ruler works fine as long as you draw from corner to corner of the overlapped square.

Sewing the seam and trimming

Sew directly on the line you marked. Use a thread color that matches one of your fabrics so any stray stitches are less visible. Sew slowly and keep the presser foot aligned with the line — this seam needs to be accurate or the strips will twist when you fold them.

Once you have sewn the seam, open the strips and check that they form one long piece with no gaps. If the corner looks loose or the seam is off-center, you can resew it. If it looks good, trim the seam allowance down to 1/4 inch using scissors or a rotary cutter and ruler. Cut away the two small triangles of fabric that stick out beyond the seam.

Press the seam open with an iron. This spreads the bulk across two layers instead of stacking it all on one side. If you are working with a delicate fabric, use a pressing cloth and low heat to avoid scorching.

Testing the joined strip before you use it

Fold the joined strip in half lengthwise, right sides together, the way you would fold it to explore it to a quilt or garment. Look at the seam from both sides. It should lie flat and smooth, with no puckering, twisting, or pulling at the corners.

If the seam puckers or the strip twists, the seam may have been sewn off the 45-degree angle. You can carefully remove the stitches, re-mark the line, and resew. Most of the time, though, a seam that looks odd when flat will settle once you press the binding onto your project.

Joining multiple strips in one session

If you are binding a large quilt or joining many strips at once, mark and sew all your seams before you trim or press. Lay out all your strips in pairs, mark the diagonal lines on each pair, and run them through the machine one after another without cutting the thread between seams. This is called chain piecing.

Once all the seams are sewn, clip the threads between the pairs, trim all the seam allowances, and press all the seams open in one batch. This approach saves time and keeps your workspace organized. You end up with one long strip ready to explore, rather than a pile of half-finished pieces.

Common mistakes and how to avoid them

The most common error is sewing a straight seam across the width of the strips instead of a diagonal one. This creates a stiff, visible line that does not bend smoothly when you fold the binding. Always mark the 45-degree line before you sew.

Another mistake is leaving the seam allowance too wide. A 1/4-inch allowance is standard for binding because it is thin enough to fold without bulk but wide enough to hold the seam find. Anything wider will create a ridge when the binding is folded and pressed.

Some people also forget to press the seam open. Pressing it to one side is faster, but it creates a thick spot that shows through the finished binding. Taking the extra minute to press it open makes a visible difference in the final result.

Frequently Asked Questions

Can I use this method with pre-cut bias tape?

Pre-cut bias tape is usually too narrow and delicate to join this way — the seam would take up most of the width. This method works best with strips you cut yourself, which are typically 2 to 2.5 inches wide. For pre-cut tape, overlap the ends slightly, fold them under, and hand-stitch them closed instead.

What if my strips are cut at different widths?

The diagonal seam method works as long as both strips are cut on the same grain. If one strip is 2 inches wide and the other is 2.5 inches wide, the seam will still sit at 45 degrees, but the overlap will be uneven. Trim both strips to the same width before you join them, or accept that the seam will be slightly off-center.

Do I need to use a special presser foot for this seam?

A standard presser foot works fine. Some people use a 1/4-inch foot to keep the seam allowance consistent, but it is not required. The most important thing is to sew directly on the marked line and keep the line straight.

Can I join bias strips made from different fabrics?

Yes. The seam will be visible on the back of the binding, but it will not affect how the binding functions. If you want the seam to be less noticeable, choose fabrics that are similar in color or pattern, or use a thread color that blends with both.

How do I know if my seam is at the correct 45-degree angle?

The easiest check is to fold the joined strip in half lengthwise and look at how it lies. If the seam is at 45 degrees, the strip will fold smoothly without twisting. If the seam is off-angle, the strip will pull or pucker at the fold. You can also use a 45-degree ruler to check the angle of the marked line before you sew.