All about half-square triangles for patchwork
HSTs: What they are and how to make them
A guide for beginners and new quilters.
If you’re new to patchwork and quilting, you may have heard about half-square triangles - HSTs for short - and wondered what the heck they are. Am I right?
The good news is that I’m here to help you out: this post explains what HSTs are, why they are so useful for quilting and how to create them using the simplest method.
What are HSTs?
Half-square triangles are EVERYWHERE in quilting! Once you know how, there are very easy to make and help you to access endless quilt pattern possibilities. If you’re a beginner quilter, I think making HSTs is the most useful method to learn, once you know the basics of patchwork and quilting.
HST stands for half-square triangle. A half-square triangle is actually a square piece of patchwork made from two pieces of fabric. The fabric pieces are triangular, but the overall shape of the patchwork piece is square, making it much easier to sew with.
First things first, why create two triangles within a square? Isn’t it easier to just cut out a triangle shape? Good question! Let me explain…
Cutting fabric with the grain or on the bias
To understand why half-square triangles are so useful and loved by quilters, it helps to know the basics about how fabric is structured and how it behaves when it’s cut.
Fabric is usually woven in a grid-like pattern:
half of the threads run vertically (up and down), parallel to the selvedge. These are the “warp” threads and together they form the ‘straight’ or ‘length’ grain.
the other half of the threads run horizontally (across), horizontal to the selvedge. These are the “weft” threads and form the ‘cross’ grain.
the straight/length grain and cross grain threads weave under and over each other to create the fabric.
For simple patchwork pieces like squares and rectangles, quilters usually cut out fabric “with the grain”. That means that they cut along the straight or cross grain of the fabric, in parallel with the vertical or horizontal threads. This is helpful because the cut out pieces keep the same grid-like structure as the fabric from which they were cut.
But if we want to create a triangle shape, we’ll have to cut diagonally across the straight and cross grains, right? This is called cutting ‘on the bias’.
Cutting across the grain of the fabric, on the bias, cuts the threads at an angle, releasing them from their tightly woven, grid-like structure. This gives the threads much more flexibility to move, making bias cuts ideal for sewing curves. Unfortunately, bias cuts for patchwork triangles tend to cause problems. Those flexible thread ends tend to stretch out of shape, making it harder to keep a triangle to the exact measurement we need for piecing patchwork shapes accurately.
Try it yourself with a scrap of fabric:
cut out a square along the grain of the fabric
gently pull on the sides of the square to test how flexible the shape is
cut the square in half, diagonally, to create two bias-cut triangles
gently pull on the corners of the triangles where the fabric has been cut and compare the stretch
see what I mean?!
Cutting on the bias is really useful for dressmaking and other types of sewing. It’s just not usually what we want for quilt-making with triangles!
Why are half-square triangles so useful for quilters?
So, what’s the alternative to bias-cut triangles? Here’s when our hero, the half-square triangle, steps in to save the day!
Half-square triangles are made with two squares of fabric. Two triangles are created at the same time and contained within a square shape. This keeps the straight and cross grain structures of the fabric intact and avoids stretch.
Let me explain how they come together…
two squares of fabric are placed together, with the ‘right’ sides of the fabrics facing each other
on the back of the top square, we mark a diagonal line
then we sew two lines of thread, each one is ¼” from the marked line – this is the standard seam allowance for patchwork
next we cut along the marked line to separate the squares into two pieces
we press the seams open or to one side (as you prefer) and, voila, we’ve made two triangles within each square shaped piece.
The magic happens because we sew the squares together BEFORE we cut them along the diagonal marked line. As we now know, cutting diagonally across the grain will create a bias cut and lots of stretch. But by sewing diagonally across the grain first, we lock the fabric threads in place so they can’t stretch and move as much.
Isn’t that clever?
Once we get the hang of the HST technique, it gets faster and easier to create accurate triangles for patchwork. There are also methods to create more HSTs at a time. But, for now, let’s perfect the two-at-a-time method.
How to make two HSTs at-a-time
Making HSTs two-at-a-time is the easiest and most common method for making patchwork triangles.
This tutorial will guide you through the process with detailed, step-by-step instructions photo slides showing each part of the process. Use these alongside any quilt pattern you are using for extra help.
1 To make two half-square triangles at a time, you will need:
A cutting mat, rotary cutter and quilting ruler(s)*
Pins
A marking tool (such as a fabric pen or pencil, tailors chalk or Hera marker - I like to use a Papermate pencil)
Two squares of fabric cut to the correct starting dimensions, according to the pattern you are using.
*in the photos below, I use a rotating cutting mat and small, square acrylic ruler to trim my HSTs. These are not essential tools: a regular cutting mat and quilting ruler will work perfectly. Just make sure that the ruler is at least a little bigger than the dimensions of the finished HST.
2 Place the two squares together, right sides facing, and mark a diagonal line on the wrong side of the top square.
3 Sew on both sides of the marked line - each line of stitches should be ¼” to either side of the marked line.
4 Cut along the marked line to separate into two pieces.
5 Press the seams open or to one side, as you prefer or as recommended if you are using a quilt pattern.
How to trim HSTs
Wait, what? Why do we need to trim HSTs?
I know, you want to start sewing those nifty triangles into a new quilt right away. But first we need to make sure they are precisely square. Trimming will fit the HSTs exactly to the dimensions you need for your patchwork pattern.
We’ve done everything we can to make sure our triangles don’t stretch out of shape. But when you press the seams of a freshly made HST, you might notice that it’s not exactly square. That’s why most modern quilt patterns and contemporary quilters will make over-sized HSTs and then trim them down to to the correct and perfectly square dimensions. Here’s how we do it:
1 Place the pressed HST on a cutting mat and use one of the diagonal lines marked on the mat to align with the seam – it’s really important as you trim to make sure that the seam stays truly diagonal
2 Trim the first side of the square
3 Continue to trim all the sides evenly to to fit the finished dimensions that you need - with each cut, double-check that the HST seamline still runs diagonally from one corner to the opposite corner.
Remember to measure twice and cut one! This is a golden rule of patchwork to avoid too many cutting and trimming mistakes.
All of my patterns include a trim of around ¼” from each side to make it easier to trim accurately. Once you’re more experienced at making HSTs and trimming them, you might find it easier to make slightly smaller HSTs and trim off less. With a little practice, you’ll have perfectly square HSTs every time!
Practice, practice, practice
Now you’ve learned all about half-square triangles and how to make them, you’ll recognise them in quilt patterns everywhere!
For more practice, try my Hadaway Quilt and Waterlilies Quilt patterns.
Once you’re feeling confident in your new skills, try making even more HSTs with my 8-at-a-time tutorial and the Minton Quilt pattern.
Kathleen x