The easiest half-rectangle triangles

Half-rectangle triangles (HRTs) are a must-know technique in modern quilt making. And no wonder, they are a great block to have in your repertoire, and add interest and open doors to many more varied shapes.

But, HRTs are the slightly more complicated cousins of the half-square triangle (HST). Where HSTs are flexible, easy-going and spontaneous, HRTs require a bit more planning and advance notice, and can feel a bit frustrating at first. But they're quite fun, too, once you get to know them ;-)

But don't worry, by the end of this tutorial, you'll be well-equipped to work with HRTs in all their many shapes and sizes.

The easiest method to trim half-rectangle triangles

There are many templates and rulers available now for trimming HRTs, but the main drawback is that they only work for a certain width to height ratio of the HRT. Some also only work on specific sizes, but even multi-size HRT rulers only work for the ratio they were designed for. So a 1:3 ruler does not work if your HRT happens to be 3" x 6", as that is a 1:2 ratio.

So you can either buy a bunch of different rulers, which might be worth it if you make a lot of HRT quilts. Or you can use the freezer paper method I'm about to show you here, which allows you to make HRTs in any ratio, and trim them easily without requiring specialty tools.

Differences between HRTs and HSTs

Before we dive into making HRTs, there are three main differences between HSTs and HRTs that you should be aware of:

The first one is that HRTs are directional. This means that while the 2-at-a-time construction method is very similar, HSTs come out non-directional and can be flipped and turned any which way to suit your quilt. The diagonal on HRTs, on the other hand, either points left or right, and flipping them does not change that direction. So you need to know which way you want it to point, before sewing that diagonal seam.
There is also more waste. Because of the rectangular shape and the fact that you can have various proportions, you may end up with a fair bit of fabric that gets trimmed off.
And lastly, trimming is not as straight forward because the diagonal seam doesn't end up exactly in the corner of the seam allowance (I explain this in more detail below).

Half-rectangle triangles are directional

Before we start on how to make them, I want to show you what I mean by "directional". This is something that keeps tripping people up (myself included), so it's important to have a closer look at it.

If you were to take an HST and turn it a quarter turn, you can make the diagonal go from pointing left, to pointing right. Keep turning it, and the colours flip while the diagonal goes from left to right, to left, to right again.

However, turning an HRT a quarter turn means it's now a horizontal block, so the proportions are now wrong for where it needs to go in your quilt. Turning it another quarter turn, so you've now turned it upside down, makes it vertical again, and flips the colours, but the diagonal is still pointing in the same direction.

So this is why it's so important to know which way the HRT needs to point before creating it.

How to make half-rectangle triangles

So, let's see how to make these fun blocks.

I'm showing you the 2-at-a-time method here for making HRTs. It is very similar to making HSTs, just with a little twist (quite literally). You can also make them 1-at-a-time, but you're still cutting a rectangle in half for each colour, so you might as well make two at once. If you only need one, put the second one away with your scraps and bonus HSTs.

Start by cutting out two rectangles (see further down for how to size those)

Mark a diagonal line on the back of one of the rectangles.

This is the important bit: the diagonal line you mark needs to be the opposite from what you want the finished HRT to have. So this means if you want your HRT to be like the first diagram above, with the diagonal going from top left to bottom right, you need to draw the diagonal from top bottom left to top right. And of course the other way to have the diagonal seam go as shown in the bottom diagram.

Lay the two rectangles right sides together on top of each other (as you would for an HST), but then twist the top rectangle with the drawn line by a quarter turn. Now the drawn diagonal matches the direction you want for the finished diagonal. This is a good place to double-check that you did draw the diagonal the right way for your desired finished HRT. If it's wrong, you can still fix it :-)

Sew a seam 1/4" away on either side of the line. The image above already shows the sewn lines because I forgot to take a picture before sewing it. But here's a close up of how that looks.

Cut the block apart on the line.

Press the two HRTs open.

How to trim half-rectangle triangles the easy way

You can also use your regular ruler, but because of the way the diagonal seam comes to lie in seam allowance (it's not exactly in the corner), it makes lining up the ruler more difficult.

I love using freezer paper, which is easy to line up, reusable, and you can do any proportion and size you want.

So, let's trim those HRTs:

Start by cutting a piece of freezer paper to the finished size of your HRT. Do not include the seam allowance in this measurement.

Place the freezer paper with the shiny side facing the right side of your HRT. Line up two opposite corners of the paper exactly on the diagonal seam. Make sure there is at least 1/4" of fabric all the way around the freezer paper. Press with an iron to adhere the freezer paper to the HRT.

Now trim the block 1/4" away from the edge of the paper. Remember, the freezer paper is the finished size of your HRT, so you need to add the seam allowances back on.

Peel away the freezer paper, and stick it to the next HRT in the same way to keep trimming. Keep making left-pointing and right-pointing HRTs until you have what you need for your project.

Easy as that!

Now, if you look at the corners of your trimmed HRT, you'll see what I meant up above when I said the diagonal doesn't line up exactly with the corner of the HRT.

This is because of the different proportions of the sides. If they are the same, as in an HST, then the diagonal is in the corner, but adding seam allowances to a rectangle means this gets shifted, and makes using a regular ruler to trim a little more difficult. This is why I love using freezer paper, because when we use the finished size, we can line it up exactly with the diagonal, and then just trim around the outside without having to worry about lining the ruler up right. It just has to be lined up 1/4" from the edge of the paper.

Calculating the starting rectangle size for HRTs

Unlike for HSTs, there is no standard formula for calculating what size rectangle you need to start with, in order to get a certain size of HRT.

Generally, for HRTs with the most common ration of 1:2 - that is 1" x 2", 2" x 4" 3" x 6" etc. - a good starting point is

For other ratios, the starting rectangle should have the same length to width ratio as the finished HRT (or very close to it). This means that for a 1:3 ratio, I would start by adding 1" to the width, and 3" to the length. However, always make a test block first with some scrap fabric, and see if that is accurate, or if you can add a little less so you have less trimming waste.