If you’re going to quilt, chances are you’re going to meet the half-square triangle—or HST—sooner rather than later!

Half-square triangles are one of the most useful building blocks in quilting. They’re simple little squares made from two triangles of fabric, but depending on how you turn and arrange them, they can create stars, pinwheels, chevrons, leaves, diamonds, and countless other quilt designs.

And today, we’re going to learn how to make them!

Even better? Once we’re finished, I have a FREE fall table runner pattern for you that will give you the perfect opportunity to put those new HST skills to work.

Meet Mr. Maple! 🍁

First Things First: What Is a Half-Square Triangle?

A half-square triangle is exactly what it sounds like: a square made from two right triangles of fabric.

The two fabrics meet diagonally from one corner of the square to the opposite corner.

Simple, right?

The tricky part isn’t understanding what an HST is. The trick is getting one that is accurate, square, and the correct size—without stretching those diagonal edges in the process.

That’s what we’re going to work on today.

My Favorite Way to Make HSTs

There are several ways to make half-square triangles, but today I’m going to show you the two-at-a-time method.

And I’m going to let you in on something else:

I like to make mine a little oversized and trim them down.

Yes, it uses a tiny bit more fabric. But especially when you’re learning, that little bit of extra fabric gives you some wiggle room.

Rather than crossing your fingers and hoping your finished HST comes out exactly the right size, we’re going to make it slightly too big and then trim it to exactly the size we need.

Accuracy without the stress. I’ll take it!

31 Threads Tip

You may see HST cutting charts that tell you to add ⅞" to your desired finished size. That method absolutely works!

But I generally prefer adding a little extra and trimming afterward. Sewing, pressing, and cutting can each introduce tiny variations, and starting oversized gives us room to correct them before the HST goes into our quilt.

Let's Make Some Half-Square Triangles!

For this example, let’s say we need a 3½" unfinished HST—meaning it will finish at 3" once it is sewn into our project.

We’ll begin with two 4" squares of contrasting fabric.

Step 1: Mark Your Diagonal

On the wrong side of the lighter fabric square, use a fabric-safe marking tool and ruler to draw a diagonal line from one corner to the opposite corner.

This line is going to be our cutting line, not our sewing line.

Step 2: Place Your Squares Right Sides Together

Place the two squares right sides together, lining up all four edges.

Pin if you like—or live dangerously. 😂

Step 3: Sew on BOTH Sides of the Line

Using a ¼" seam allowance, sew a straight line ¼" away from the marked line.

Then turn the unit around and sew another seam ¼" away from the marked line on the other side.

When you’re finished, your marked diagonal should run right between your two lines of stitching.

Step 4: Cut Them Apart

Now cut directly along the diagonal line you originally marked.

And look at that—you just made TWO half-square triangle units!

Step 5: Press

Open each HST and press the seam.

Depending on the project, I may press toward the darker fabric or press the seam open. Follow the pressing directions in your pattern when they’re provided.

One important thing here:

Press—don’t aggressively iron back and forth.

Those diagonal edges are cut on the bias, which means they can stretch more easily than fabric cut along the straight grain. Let the iron do the work instead of pushing and pulling the fabric around.

Sound familiar? We talked all about pressing vs. ironing in last week's blog!

Now for the Secret Weapon: Trim Them!

This is the step that can take your HSTs from pretty close to exactly right.

Grab a square quilting ruler with a 45-degree diagonal line.

Before trimming, make sure your HST is large enough to reach the size you need on all four sides.

For our example, we're trimming to 3½" square.

Step 1

Place the ruler over your HST and line the ruler’s 45-degree diagonal line directly on the seam between your two fabrics.

Make sure you have a little excess fabric extending beyond the 3½" marks.

Trim the top and right sides.

Step 2

Rotate the HST 180 degrees.

Again, place the 45-degree line directly along your diagonal seam.

This time, line the two freshly trimmed edges up with the 3½" marks on your ruler.

Trim the remaining two sides.

Now measure it.

3½" × 3½".

Beautiful diagonal seam. Square corners. Correct size.

THAT is an HST ready to go into a quilt!

A Few Common HST Troubleshooting Tips

If your HSTs aren't cooperating, don't panic. Usually the problem comes down to one of a few things.

My HST is too small.
Start with slightly larger squares next time. Remember, our goal with this method is to have a little extra fabric available to trim away.

My diagonal seam isn't running perfectly from corner to corner.
When trimming, make sure the 45-degree line on your ruler stays directly on the seam during both trims.

My HST looks stretched or wavy.
Be gentle when pressing. Those diagonal edges are bias edges and can stretch if they're pulled or pushed around too much.

My points disappear when I sew my blocks together.
Remember that the outer ¼" of your unfinished HST becomes the seam allowance. Your diagonal seam should meet the corner of the unfinished unit so that once it's sewn into the quilt, your point lands where it should.

And finally...

My HST isn't perfect.

Welcome to quilting. 😂

Accuracy matters, but perfection isn't the price of admission. Learn what caused the problem, make an adjustment on the next one, and keep sewing.

That's how we get better.

Now Let's Put Those HSTs to Work!

You've learned what an HST is.

You've sewn them.

You've pressed them.

You've trimmed them.

So I think you deserve a project!

Meet Mr. Maple, my FREE fall table runner pattern. 🍁

Mr. Maple uses half-square triangles to create three maple leaf blocks in warm fall colors. It's a manageable project, a great way to practice the skill we just learned, and—most importantly—it's really cute sitting on a fall table!

Get the FREE Mr. Maple Pattern

Ready to make your own?

GET YOUR FREE PATTERN HERE! ⬇️

https://31threadsquiltco.com/pages/free-maple-leaf-pattern-table-runner

Grab some fabric from your stash, choose three colors for your leaves, and start making those HSTs.

And when you finish your Mr. Maple runner, I want to see it! Tag 31 Threads Quilt Co. when you share your project so I can cheer you on.

Happy quilting!

— Crystal