Half-Square Triangles (HSTs) are a common unit used in so many quilts from traditional to modern. Some quilts are made entirely from HSTs! In this post I want to show you how I square them up. This method is quick, easy and accurate!
First you need two squares cut 1″ larger than the finished size of the half-square triangle. The finished size is the size the unit will be once it is sewn into your quilt. For this example I need HSTs that finish at 2.5″ so I have cut my squares 3.5″.
Next, layer the two squares right sides together. I am using a 1/4″ seam allowance guide from Creative Grids. I love their rulers because they have a bit of texture on the wrong side that prevents the ruler and/or fabric from shifting once you apply pressure to the ruler. You can see I have placed the ruler from corner to corner on my square with each corner being right in the middle of the channel on the ruler.
Use a mechanical pencil to draw a diagonal line on each side of the ruler. These lines will be 1/4″ from the exact diagonal of your square. It is important to use a mechanical pencil because that will give you a nice fine line. A dull regular pencil or some fabric marking pens make a thicker line which is not desirable for our purposes.
Now you will sew just inside each of your drawn lines. You can see in the photo that my lines of stitching are truly right next to the drawn lines.
Place a ruler from corner to corner and use a rotary cutter to cut the square in half on the diagonal.
You now have two half-square triangles ready to be squared up.
First you need to prepare your square ruler. Place a piece of painters tape diagonally on your ruler, connecting the marks along the edges of your ruler that correspond to the unfinished size of your your units. Since my units need to finish at 2.5″ they need to be squared up to 3″. Always add 1/2″ to the finished size of your unit to arrive at the size to square up your unit to.
This picture shows the ruler from the wrong side with the tape lined up exactly at the 3″ marks.
Fold any extra tape to the wrong side of the ruler.
Place the ruler on your half square triangle placing the tape right along the row of stitching. You can just see the stitches right next to the tape in the photo.
Next, move the ruler ever so slightly to just cover the row of stitching with the tape. You only want to move the ruler enough so the stitches just disappear.
Now use your rotary cutter to trim off the small amount of fabric sticking out beyond the right side and top of the ruler.
Place the triangle so it is sitting on one of its short sides. Place the diagonal line on your ruler on the line of stitching and place the bottom edge of the ruler along the bottom of the triangle. If your ruler doesn’t have a diagonal line, no worries, just make sure the row of stitching lines up with the intersections of the inch marks.
The key to cutting off enough of the corner is to make sure the right edge of the ruler is lined up right where the line of stitches starts on the edge of the fabric. Notice the ruler is lined up properly because none of the stitches are showing, meaning they are perfectly covered with the diagonal line of the ruler.
Use a rotary cutter to trim off the triangle of fabric extending beyond the ruler.
Flip the triangle over and line up the other short edge with your ruler as before. Trim off the point as well.
The HST is fully trimmed and ready to be pressed. Place it on your pressing surface. The side that is facing up will be the side that the seam allowance is pressed toward.
Since I had the green square facing up on my pressing surface that is the direction my seam is pressed toward.
A perfectly pressed HST unit!
And a perfect 3″ square!
I love this technique because it is only necessary to trim two sides rather than all four. This results in a perfect square every time! I suggest trying this technique with two scrap squares of fabric before you jump into a big project. You want to make sure you are sliding the row of stitching just under the tape. If you slide it too far your finished unit will be a little bit too big. If you don’t slide it under enough the unit will be too small. The amount you slide the ruler to cover the stitches is slight but you want to get it right BEFORE you start cutting all the fabric for your project!
Once you are comfortable with the technique there are two spots where you can cut with your scissors rather than a ruler and cutter. First, after you sew the two lines of stitching you can cut the squares apart with your scissors. Since you already sewed those seams the size of the seam allowance along that edge is irrelevant. The other time you can use scissors is to cut off the tiny triangles at each end of the row of stitching. Use a ruler and cutter the first few times to familiarize yourself with how much of a triangle you are cutting off, then feel free to use your scissors. I find that most of my students don’t cut off enough at the points. The reason we trim those points is so they don’t interfere with our presser foot in future seams, as well as eliminating bulk at that point which is an important consideration for when you quilt your quilt.
Good luck squaring up your half-square triangles! I hope this tutorial is helpful for you!
I hope to see you again soon!