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How to Trim a Half Square Triangle

Half square triangles, or HSTs for people who like acronyms, are one of the building blocks of quilting. This whole quilt is half square triangles with squares and strips.

My quilt, Good Morning Sunshine, rocks the half square triangles

I've found tons of online instruction about how to sew half square triangles - individually and in twos, fours, eights and even sixteens but then they get to the step where they just say "trim" and I didn't really know what to do. In case you are having the same problem, I thought I'd show you what you need to know. (If it seems obvious to you, my mother taught me that sometimes it's better to just not say anything at all.)

First, here is a two second primer about how to sew a half square triangle.

  1. Take two squares, ½ to ¾ inches bigger than you want your final square to be, and match them up right side to right side.

  2. Draw or iron a line on the diagonal.

  3. Sew on both sides of the diagonal line, ¼" away from the diagonal line.

  4. Cut along the center diagonal line, in between the seams, and press open.

Trimming your half square triangle

Now, because you've used a cool trick to make your HST, you'll need to trim it to get it down the exact, perfect size. There are two things you want to have happen:

  1. The seam should be perfectly in the corner.

  2. The square should be the correct size.

At first, I didn't understand how important it was to have the diagonal line hit right in the corner and my stuff was wonky. (Not that wonky is bad, if that is what you want, but if it's not what you were aiming for, it's at best disappointing!)

Step #1

Place the seam so it lines up with the 45 degree diagonal on your cutting mat with the top corner in line with inch marker. (If your mat doesn't have diagonal lines, buy a new mat or find a friend who is willing to share. Better yet, come sew at Hello Stitch where they've got them all over the place! I'm sure there are other solutions but, honestly, I think you'll thank me for pushing you to use a better mat.) If you have a rotating mat, use it, it's the bomb and this is just what you bought it for.

Getting the half square triangle lined up is the most important step

Step #2

Align your ruler, at the size you want to cut the half square triangle, at the line on the mat that will be the left edge of your square. In the photo below, I'm making a 2 ½" square. Trim the right edge. Pick up your ruler very carefully, ideally you don't want to move the HST you are trimming. If it moves, realign it with the diagonal.

Measure and trim the half square triangle

Step #3

Rotate the mat or your body so you can trim the next edge. Be sure to check that the center seam is still on the diagonal, measure and trim. Keep doing this until you've done all the sides that need to be trimmed.

Rotate, measure, trim, repeat

Step #4

Step back and admire your work. People who don't quilt don't understand what it takes to achieve perfect points. Take a picture and text it to that friend who talked you into getting into quilting!

A nicely trimmed half square triangle

Step #5

Now, do the next one. What is it about half square triangles that seems to require you to do thousands of them at a time?

Step #6

Have fun!


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