How to Trim Half Rectangle Triangles

Written by Brittany Lloyd

I don't know about you, but I'm absolutely obsessed with the look of the half rectangle triangle (HRT). I've procrastinated learning the block for SO long and now that I've mastered it, I'm not sure why I waited this long! Like the half square triangle (HST), you can achieve so many fun and bold "looks" with this block; but the HST and HRT have a few BIG differences. Let's take a closer look.

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A HST seam is diagonal from corner to corner, creating a 45 degree angle. A HRT's seam is NOT 45 degrees because it is a rectangle...so the ratio of the rectangle edges determine the angle of that diagonal seam.

Because of this you'll still trim your HRT unit with the diagonal seam in mind, but instead of trimming to position the diagonal seams exactly in the corners of your piece, they'll be offset just a little bit.

Take a look at the photo below. This unit is already trimmed; you can see that the diagonal seam intersects 1/4" in from the edges of the unit. This means that when you go to sew a 1/4" seam allowance on all four edges of the unit to sew it into your quilt, your diagonal seams will then right exactly from corner to corner of the block. That's what gives us our perfectly crisp points.

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So how do we go about making this happen? I've found that marking temporary dots on my ruler really helps me visualize this process. I use a regular permanent marker to mark the top of the ruler and then when I'm finished, I'll take it off with some rubbing alcohol. You're welcome to skip marking your ruler, but having those visual landmarks really do make this process simple and easy!

Mark guide dots 1/4" in from both sides of each corner of the unfinished unit measurements. So if you are trimming the HRT unit to measure 2-1/2" x 4-1/2", then you'll create dots at

1/4" x 1/4" (Right HRT)
2-1/4" x 4-1/4" (Right HRT)
2-1/4" x 1/4" (Left HRT)
1/4" x 4-1/4" (Left HRT)

Note that this is to trim both Left HRTs and Right HRTs. Another big difference between HSTs and HRTs is that they are not rotate-able. Since a HST is a square, you can rotate it around and orient the triangle in any position. With the HRT, since it's a rectangle, you can hypothetically only rotate the piece in two positions while still keeping the overall position of the unit the same. So let's look at trimming a Right HRT first.

1. Align the ruler so the diagonal seam is underneath both Right HRT dots. Ensure the unit extends past the measurement that you are trimming to on the bottom and left edges of the unit. Trim the right and top sides.

2. Rotate the unit 180 degrees and align the ruler so the diagonal seam is underneath both Right HRT dots. Ensure the unit aligns evenly with your desired unfinished measurement. Trim the right and top edges.

Trimming the Left HRT unit is the exact same process, but slightly different since the seam is opposite.

1. Align the ruler so the diagonal seam is underneath both Left HRT dots. Ensure the unit extends past the measurement that you are trimming to on the bottom and left edges of the unit. Trim the right and top sides.

2. Rotate the unit 180 degrees and align the ruler so the diagonal seam is underneath both Left HRT dots. Ensure the unit aligns evenly with your desired unfinished measurement. Trim the right and top edges.

That wasn't so bad, was it? Once you understand how HRTs work, trimming really does get easier with practice!

And if specialty rulers are your thing, you can also use Bloc Loc HRT rulers! You just have to make sure to purchase one that is the 2:1 ratio for Mosaic star. We actually have these rulers in our shop! You choose from the Mini or Large HRT ruler. The Mini is exactly the size that you need but the Large can trim HRTs with a 2:1 ratio up to a 4.5" x 8.5" size!

Need further trimming advice? Check out these trimming tutorials:

And if you want to try all of these techniques in one place, be on the lookout for the Mosaic Star pattern release on November 17th!