How to Sew a Half Square Triangle

Just recently, I set out on a quest to get back to the basics of sewing and quilting. I was having some trouble with points & seams lining up, and I wasn’t improving. My first task was perfecting my quarter inch seam allowance, but that is a story for another day. 🙂

(This pillow cover is listed in my Etsy Shop.)

Second on my list was perfecting the basic half square triangle. This simple block is used in SO MANY patterns. You can take the same stack of half-square triangles and arrange them in a vast array of patterns and end up with so many options to create a quilt.

My goal was to simply sew an 18 inch pillow cover. This took twenty-five blocks, and I created 4 1/4 inch blocks for this project.

This project was created using Kona’s Color of the year — pink flamingo. And I used Kona white for the other half of the block. This new pink solid is a really great shade of pink. Not to dull. Not to bright. And it has just a hint of coral.

1. In order to finish a 4 1/4 half-square triangle block, begin by cutting five inch squares. This tutorial includes trimming down your blocks at the end.

If you aren’t sure how to cut five inch squares, start with this tutorial here.

2. Draw a line from one corner of the square to the other. If I have the option, I almost always draw on the darker fabric. I read recently in an old quilting book to lay your fabric on sand paper if you have trouble with it shifting while marking your fabric.

My husband recently bought me this quilt pencil kit off Amazon for all my marking needs. 🙂

3. Stitch a seam a quarter of an inch down either side of your line.

4. Using your ruler and rotary cutter, cut those seams apart using your line as a guide.

5. Iron your block. For this project I ironed all my seams toward the darker fabric.

For durability and for the purpose of nesting the seams, I stuck with ironing toward the dark fabric for this entire project. I did have a bit of trouble with bulky seams at the end but not enough trouble that I’d advise otherwise at this point.

6. Trim your half-square triangle block down to a 4 1/4 block.

I know this may appear to be wasting fabric. In the end, though, you are essentially saving yourself some frustration. Half square triangles are hard to sew perfectly without trimming them down. I HIGHLY recommend sewing them oversized and trimming them.

You need to have a ruler or square template with a diagonal line as seen below. Line up that diagonal with the seam of your half-square triangle. Trim the right side and the top.

Flip your square around and trim the other side. Note how the left side and bottom side of the half-square triangle are lined up with the 4 1/4 mark on the ruler.

And you are left with PERFECT half-square triangle blocks.

Sew these blocks into a few rows and choose some backing, and you have the perfect patchwork pillow cover. I use this tutorial to install a zipper.

The back of this pillow is Paper Eponine by Amy Sinibaldi. It’s a precious fabric with dainty blue and pink flowers. I also just listed this pillow cover in my Etsy Shop. I love this cover and this pink is absolutely perfect, but in a home with three little boys, I don’t have a bedroom to decorate with pink right now.

(Please note that this post contains affiliate links.)

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