The diagonal seam method joins two bias strips with almost no thickness

When you need a long run of bias binding but your fabric only gives you short strips, you join them by sewing the ends together at a 45-degree angle. This diagonal seam — called a bias seam or diagonal join — creates a thin, flat connection that won't bunch when you fold the binding or press it. The seam itself becomes part of the binding, so it disappears into the fold.

The method works because bias strips are already cut at 45 degrees to the grain. When you align two strips end-to-end and sew them at that same angle, the grain lines up and the seam sits flat. If you sewed them straight across instead, you'd create a thick lump that shows through on the finished edge.

You'll need two bias strips, a ruler, a pencil or fabric marker, pins, and a sewing machine. The whole process takes about five minutes per join once you've done it once or twice.

Key Takeaways

  • Bias strips join with a diagonal seam that runs at 45 degrees to the strip edges, not perpendicular to them.
  • Mark the seam line with a pencil or fabric marker before pinning so the stitches follow the correct angle.
  • Trim the seam allowance to 1/4 inch after sewing, then press it open flat to keep bulk out of the binding.
  • Test the join by folding the finished strip in half lengthwise — the seam should lie flat with no ridge.

Lay out the strips in an L shape and mark the seam line

Place one bias strip on your work surface with the right side up. Lay the second strip on top of it, also right side up, so the end of the second strip crosses the end of the first strip at a right angle — they form an L or a cross. The strips should overlap by about an inch.

Now rotate the top strip 45 degrees so it sits diagonally across the bottom strip. The two strips should now form a straight line when you look at them from above, with one strip running left-to-right and the other running at a 45-degree angle through it. This is the position you'll sew.

Use a ruler and a pencil or fabric marker to draw a line across the overlap where the two strips meet at that 45-degree angle. The line should run from the upper left corner of the overlap to the lower right corner (or upper right to lower left — either direction works as long as it's diagonal). This line is your seam guide.

Pin and sew along the marked line

Flip the whole assembly over so the bottom strip is now on top. You should see the pencil line on the back of the top strip. Pin through both layers along the line, placing pins every inch or so. The pins hold the strips in the correct position while you sew.

Set your sewing machine to a standard stitch length (about 2.5 mm) and thread it with thread that matches your fabric. Sew along the pencil line, removing pins as you reach them. Sew all the way across the overlap, from one edge to the other. Backstitch at the start and end to lock the thread.

After sewing, remove the pins and check that the seam is straight and the stitches are even. If the line wobbles or skips, you can pick out the stitches and try again — this is normal on your first few joins.

Trim the seam allowance and press it open

Lay the joined strips flat on your cutting mat with the seam facing up. Use a ruler and a rotary cutter to trim the seam allowance down to 1/4 inch on both sides of the seam line. You'll cut away the excess fabric that extends beyond the 1/4-inch margin. This step is what keeps the join from creating a thick ridge in the finished binding.

Flip the strips over and press the seam open with a hot iron. The seam should lie flat and split evenly between the two strips — you're not pressing it to one side, but opening it up like a book. This open seam is what makes the join invisible when the binding is folded and sewn to an edge.

Once the seam is pressed, the two strips are now one long strip. You can continue cutting bias strips from your fabric and joining them the same way until you have the length you need.

Check that the join lies flat when the binding is folded

Before you use the joined strip as binding, fold it in half lengthwise with wrong sides together and press it. This is how bias binding sits when it's sewn to a quilt or garment edge. Run your finger along the seam — it should feel smooth and flat, with no bump or ridge. If you feel a lump, the seam allowance is too wide or wasn't pressed open far enough.

If the seam does create a ridge, you can unfold the strip, press the seam open again with more heat and pressure, and then re-fold it. Sometimes a second pressing flattens it completely. If the ridge persists, the seam allowance may need to be trimmed narrower — trim it to 1/8 inch and press again.

Common mistakes and how to avoid them

The most common error is sewing the strips together straight across instead of at an angle. This creates a thick seam that shows through the binding. If you catch this before you've cut the bias strips, you can pick out the stitches and re-sew at the correct angle. If you've already cut the joined strip, you'll need to undo the join and start over.

Another frequent mistake is not trimming the seam allowance. A full 1/2-inch seam allowance on both sides of the line creates a 1-inch-thick lump in the binding. Always trim to 1/4 inch after sewing. The third mistake is pressing the seam to one side instead of open. An open seam distributes the bulk evenly and disappears into the fold; a seam pressed to one side stays visible.

If your pencil line isn't perfectly straight, the seam will angle slightly and the joined strip won't be perfectly rectangular. This usually doesn't matter — the binding will still work — but if it bothers you, use a ruler and a sharp pencil to draw the line carefully before you pin.

Joining strips of different widths or lengths

If you're joining strips that are different widths — for example, a 2-inch strip to a 2.5-inch strip — the diagonal seam still works, but the joined strip will have a step in it. Trim both strips to the same width before you join them, or accept that the binding will be slightly wider at one end. Most sewers trim to match.

If you're joining a short strip to a very long one, the same method applies. The angle of the seam stays the same regardless of length. Just make sure the overlap is at least an inch so you have room to sew without hitting the end of the strip.

Frequently Asked Questions

Can I use a 1/2-inch seam allowance instead of 1/4 inch?

You can, but the seam will be thicker and more likely to show through the binding when it's folded and pressed. Start with 1/4 inch. If the seam still creates a ridge after pressing, trim it to 1/8 inch rather than making it wider.

What angle should the seam be if my bias strips are cut at a different angle?

The seam should always run at the same angle as the grain of your bias strips. If your strips are cut at 45 degrees, sew at 45 degrees. If they're cut at a different angle (which is rare), match that angle. The point is that the seam grain should align with the strip grain.

Do I have to press the seam open, or can I press it to one side?

Pressing it open is standard and keeps bulk out of the binding. Pressing it to one side works in a pinch, but the seam will be thicker and may show through when the binding is folded. Open is better.

What if I sew the seam crooked and it doesn't line up?

Pick out the stitches with a seam ripper, re-mark the line if needed, and sew again. Fabric is forgiving — you can redo a seam as many times as you need. Once you've done three or four joins, your hand will know the angle and you'll sew straighter.

Can I chain-piece multiple joins together to save time?

Yes. After sewing the first join, don't cut the thread. Instead, position the next two strips in the L shape, sew that join, and repeat. When you're done, cut the thread between each join and trim the seam allowances. Chain-piecing saves thread and keeps you in a rhythm.