Piecing With Diagonal Seams

Diagonal seams. Most quilts have them. Whether it’s a Half-Square-Triangle unit, a Flying Geese unit, a Square-in-a-Square block, a star, or any of a thousand blocks you might choose, having a few options for sewing a diagonal seam can speed the process along. And, hey, we like to get things done, right?

There are basically two methods to use when sewing a diagonal seam. Marked or unmarked.

Let’s start with “Marked”.

Now, you can use some type of marking tool, such as, a pencil (I recommend using a mechanical pencil with a graphite or chalk lead made for fabrics), a pen with ink that disappears with heat, or a marker with ink that disappears with air or water. Something to draw a definitive line on which to stitch. I recommend placing your fabric on a sandpaper surface for marking. The sandpaper keeps the fabric from shifting as you draw.

Sometimes your pattern will call for a line drawn exactly on a 45 degree diagonal from one corner to the other. Any ruler that is at least as long as the diagonal measurement from corner to corner will work.

Depending on your pattern instructions, you either stitch directly on the line or stitch 1/4″ away on both sides of the line.

Other times, two lines are drawn, one on either side of the corner-to-corner diagonal. This photo shows a ruler called a Quick Quarter being used.  The ruler has openings down the center of it. You line up the center openings from corner to corner. Then, draw a line on either side of the ruler.


Place a marked square on top of a piece of fabric of equal size, right sides together.
When each line is sewn, you cut from corner to corner to separate the units.
  Two Half-Square-Triangle units, done!

Another method for “marking” a diagonal line, is to simply fold the square exactly in half diagonally and press the fold. Either finger press (which I don’t recommend because it can stretch the fabric) of use an iron. The hot  iron can simply be set down on the fold. This gives a nice crease without stretching.  You can then, unfold the square, place it on another square, right sides together, and stitch along the folded line. Many people have success with this marking method. I have not, because the fold line, for me, can be difficult to follow, especially if the fabric lays too flat and the fold virtually disappears before my eyes. I do recommend trying it. It may work for you, it may not, but you won’t know unless you try.

Now, let’s talk “Unmarked”. This is my method, and although it is called “unmarked”, there actually is a little bit of marking involved. However, it’s not the fabric that is marked, but, the bed of your sewing machine. So, you do have to mark, but, it’s only once, then you can stitch away!

I’ve detailed this method in a post called “Less Marking, More Sewing“.  I mark three lines (the far left one is close to the edge of the tape, so it may be hard to see), and, depending on where my diagonal seam needs to be, use one of these lines as my guide. Check out “Less Marking, More Sewing”. You’ll be glad you did!

I use either one of these methods, “marked” or “unmarked”, when a pattern calls for a diagonal seam. It just depends on the size of the seam to be sewn. 

My “Unmarked” method, for my machine bed, works best for a diagonal seam that is no longer than 7″, or a square that is no larger than 5″. If your sewing machine is set into your sewing table, you might be able to use squares that are larger than 5″. If the diagonal seam I need to sew is longer than 7″, I mark it.

The “Unmarked” method works great for sewing small diagonal seams, such as, binding or border strips.
When sewing the “tooth” units for Just Buzzing Through, keep the corner of the bottom fabric lined up with the center line on the machine bed.
If the bottom corner is difficult to see, bend up the top piece so that you can see the corner, then fold it down into place just before it goes under the presser foot.
Or, you can make a tiny mark on the top piece, at the edge of the underneath corner. Just to give you a visual “corner”.

Marked or unmarked. The choice is yours.

Try both methods and see what works for you.

I hope these tips have been helpful. But, if you have any questions, please email me at diane@lookoutmountainquilter.com. I will answer as soon as possible.

Happy quilting!