Ever wondered how to sew and trim half rectangle triangle (HRT) blocks? Well you’ve come to the right place! Let me show you how I like to do them.
Half Rectangle Triangles add another fun shape to your quilt designing arsenal. You can make cool, elongated diamonds and zigzags and parallelograms and…long spikey things. The list goes on and on.
What exactly is a Half Rectangle Triangle (or a half square rectangle as I often accidentally call them)?
If you’ve made half square triangles (HSTs) before you might be thinking, no sweat, an HRT is just a stretched out HST. And you’d be right. After all, a square is just a rectangle with 4 equal sides.
Being square, you can rotate an HST 90 degrees and still have an identical shape but with the diagonal seam reversed.
Above: An HST block rotated 90 degrees each time – every diagonal option and colour placement option can be achieved with the one block. Oh yeah.
If you rotate a half rectangle triangle 90 degrees to change the direction of the diagonal you will NOT have the same shape. Your long, tall rectangle will now be short and wide, or vice versa.
Above: The HRT’s diagonal direction will determine the layouts. Here the 2 possible HRT blocks are rotated through 90 degrees. You can’t get every layout from only one HRT block.
All that to say – when making half rectangle triangles you have to pay attention to the direction you need your diagonal to face. This isn’t tricky, but something to keep in mind before you race out and make a whole bunch.
How I like to sew Half Rectangle Triangles
My preferred half rectangle triangle method goes something like this:
1. Take two rectangles
2. Cut the rectangles in half diagonally to give triangles
3. Sew triangles together
4. Trim to size.
I’ve seen other ways that are more HST-like in their process. Two rectangles have quarter inch offset diagonals drawn on them in opposite orientations, and these are then matched right sides together, opposite corners together. Much like an HST, a seam is sewn a quarter inch each side of the diagonal and the units are cut apart and pressed open. It’s very clever because what you’re sewing doesn’t look like an HRT until the last step. (Added benefit: no requirement to handle any bias cut edges while sewing.)
The downside, for me, is that the set up is quite finicky (offsetting the diagonal and matching the lines). Given the sewing time I have usually coincides with my wine drinking time, I like to keep my sewing methods as straight forward as possible so I have fewer opportunities to make mistakes. The method I follow is very “make it like you see it” and it suits me better. Feel free to choose whatever suits you!
One other thing I like about this method is that it produces the half rectangle triangles one unit at a time – so I can mix and match halves as needed, I don’t have to make 2 of each same fabric pairing. I can also lay out my triangles on my design wall and swap around as needed, which I can’t do if I use the HST-like method.
That’s a big plus when making a quilt like my Slip Stitch quilt. I laid it all out on my design wall before I sewed my HRTs together, then took a step back and considered my colour placement. I could move the rows up and down as I wanted because the triangles hadn’t yet been joined to their partners.
So let’s get on with the sewing, shall we?
How to sew Half Rectangle Triangles in any size
The longer and skinnier the half rectangle triangle, the trickier it is to trim. The squat, wider types are easier. I thought I’d go with a harder example, so it can only get better from there!
Above: the guy on the left is easier to trim than the guy on the right.
I’ll make a unit with each diagonal orientation so you can see how they both work.
I’ve taken two sets of low-volume and purple rectangles that are 2-1/2″ x 7″ (more on initial rectangle size later).
Each pair of rectangles has a diagonal cut in the direction required (as determined by your design).
Each pair of rectangles can made 2 HRT units, but I’ve set aside the the extra triangles for the time being.
First sew the halves together – the important thing to remember here is to offset the halves by a quarter inch.
The easy way to remember this is that the pointiest points need to point over the edge. Silly, I know, but it’s how I remember these things (perhaps explained by my wine comment, above).
You can see below the triangles have been matched right sides together, but offset from their halves by a quarter inch (I do this by eye, I don’t measure it). The pointier points (i.e. the more acute or sharp angles) are past the edge of the wider (more blunt) angles, both top and bottom.
You can pin (or not) and then sew a quarter inch seam to join the two halves.
Press the units as your design requires (you may want to nest seams, or prefer the flexibility that open seams give).
Now we’re ready to trim up. These rectangles will yield un-finished HRT units that are 2″ x 5″. Once finished (sewn together), they will measure 1.5″ x 4.5″ (a ratio of 1:3).
First the unit needs to have the longest edges trimmed to give the required width. Here I’ve trimmed the first unit to 2″ wide:
Grab your ruler – preferably one which has a corner with quarter inch markings that intersect (such as this Creative Grids* one):
Start at one of the corners with a diagonal. Line up one edge of the ruler with the long edge of the HRT. Then slowly slide the ruler (keeping the ruler/block edges lined up) until you find the spot where the diagonal is exactly a quarter inch from the long edge.
Taking a closer look at the diagonal, below, you can see it falls directly under the intersecting quarter inch marks on the ruler. And if the diagonal is exactly a quarter inch from each side then it will make a nice point when it is sewn to its neighbouring blocks. So we trim here.
Now the half rectangle triangle unit is rotated around to trim the opposite end. Again, the ruler is kept flush with the long edge of the HRT and the diagonal must fall under the intersection of the quarter inch ruler marks. We can also see that the HRT is 5″ long, as required.
The process is the same for the other type of half rectangle triangle unit (the one with its diagonal sloping in the other direction):
First, trim the long edges:
Then find the spot where the diagonal matches the quarter inch marks and trim:
Then flip the HRT around to the other side and…oh no: I’ve measured my block to be 5″ long, but the diagonal isn’t falling right under the intersection of the quarter inch marks.
This is why the longer, skinnier blocks can be trickier to trim. Because that angle is so much sharper than the fatter units, it’s easier for it to be a touch off.
An additional, trouble-shooting step I like to include before trimming long HRTs is to centre the unit on my cutting board such that it spans the length I need. Below you can see my scissor tips are marking out 5″ and I’ve centred the unit on this, eyeballing the ends so even amounts of “point” overhang the 5″. Then you can quickly check the diagonal at each end before trimming and move the unit a little if a bit of fudging is required.
Half Rectangle Triangle Calculator
A note on calculating the starting size of your rectangles for your half rectangle triangles.
HRTs with a side ratio of 1:2 are not only easier to trim, they are also easier to calculate starting rectangle sizes for.
Above: finished HRT blocks with ratios of 1:2
The dimensions of the oversized starting rectangles are:
(Finished Width + 1″) by (Finished Length + 2″).
So if you want a half rectangle triangle that is 2″ x 4″ when it is sewn into a quilt, you will need two starting rectangles that are (2 + 1) by (4 +2) or 3″ x 6″. Easy peasy. Thank you, half rectangle triangle calculator.
If you’re allergic to math then pop your email in the box below and I will send you a PDF chart with the sizes needed to create 1:2 ratio HRTs from teeny weeny to big and bold. I promise I didn’t drink any wine while I did these calculations. ;)
That will also sign you up to my newsletter, which means you won’t miss out on new tutorials or pattern releases. Yay!
When making non-1:2 HRTs, you’ll need to make a guess as to the size of the starting rectangles and go from there. I usually apply the same formula (add 1″ to the finished width and 2″ to the finished length) and make a test block.
Then it’s just a matter of testing and adjusting. If the block is too long when you trim, reduce the length and try again. This was exactly how I tested for the starting rectangles for my 1-1/2″ x 4-1/2″ (finished) block.
Alright, if you’re still reading then you are definitely ready to get out there and sew up some half rectangle triangle quilt patterns. As well as the patterns in my shop, I also have a free scrappy half rectangle triangle quilt pattern over in this post here, if you’re interested.
Hopefully this post has helped start you on a half rectangle triangle journey. Be sure to send me an email if the 1:2 HRT size you need isn’t included on the PDF download – I’ll be happy to help you with that.