What is a half square triangle?

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Cut and Sew Cheat Sheet for HSTs – two or eight at a time
How to make HSTs two at a time
How to make HSTs eight at a time (Magic 8)

What is an HST?

Many quilt patterns start with making a bunch of half square triangles (HSTs) or more properly, half square triangle units.

Technically an HST is just a triangle that fits into half of a square. I think that makes it a right isoceles triangle.

But in quilting it generally refers to a square made up of two triangles or a HST unit – or HST for short.

Why make HSTs from squares?

The reason quilters make them in these units is because of the difficulty in working with bias edges. Bias edges are those that are cut diagonally across the fabric. These types of edges do not have the strength of either the crosswise or lengthwise threads holding them, so they stretch easily. Once they stretch they rarely shrink back down to size, making your quilt all wonky.

If you cut a bunch of triangles and then sew them together there’s a good chance you will have some stretching on the bias.

By making them in HST units, we sew a seam on the diagonal first, and then cut on the diagonal. This means that the seam can help keep that diagonal bias edge from stretching.

Making 2 HST units at a time

What size square to start with

To make half square triangles two at a time, you want to start with two squares of fabric that are  ⅞” larger than the finished size of your HST unit.  Finished size is the size of the square that will be showing on the front of your quilt – without all the seam allowances.

To trim or not to trim…

The charts below indicate a standard size square to cut and another column entitled ‘if you like to trim’. By starting with the standard size square (the one that adds ⅞”) you should get an HST unit that is exactly the size you need.  

If you are just starting out it may be harder to mark the center sewing line exactly through the corners, or to sew an accurate ¼” seam from either side of the line.  If this is the case, when you press open your HSTs, the seams may not line up perfectly in the corners or be a bit crooked.  For this reason, many people prefer to start with a slightly larger square that then then trim down to the proper size.  This allows a little more leeway in marking and sewing exactly.  This also takes more time and more fabric. 

If you are just starting out, I suggest that you try one square with the standard ⅞” measurement and see how it comes out.  If it is not square or the seams don’t line up at the corners, then go ahead and start with the slightly bigger squares and trim them.  This is not a failure on your part!  In fact many patterns call for creating and trimming all the HST units in this way, and many experienced quilters prefer this method.

HST cutting square measurement chart

Mark the diagonal

Mark the one diagonal line on one of the squares.  Use a light touch with the marking pencil so that you don’t stretch the bias as you are marking.  Make sure the line goes straight through the corners, not on one side or the other.

Right sides together

Put your two squares right sides together and match them closely on the sides. Matching the edges closely, especially at the corners is an important part of making the HST units square at the end.  It may help to pin on either side of the diagonal line so your carefully matched squares don’t get disturbed on their way to the sewing machine.

Pinning half square triangle units


Sew a scant ¼ inch seam on both sides of the line. Scant is just an old word for ‘not quite the whole amount’. So a scant 1/4 inch seam is just slightly under a 1/4 inch. About the width of your stitching line or a couple threads. Because you will not be sewing a 1/4 inch from the edge of the fabric, but from the line you marked, this is where it helps to have a presser foot that indicates an accurate 1/4″ seam instead of relying on a seam guide.


Cut the square in half on the diagonal line. Be gentle with the squares as you cut them as you are now creating a stretchy bias edge.

HST unit cut apart to two


Even though we’ve put a seam on the diagonal, we still want to be careful when pressing the seam open so that we don’t stretch the bias.  The best way to do this is to first set all the seams.  

To set the seam, just press the seam as it is.  Lay your unit on the ironing board with the darker side to the top (if you want to press your seams to the darker side).  Then just zap it with the iron.

After it cools slightly use your fingers to gently open the square and finger press the seam open and in straight line before zapping it with the iron again.

Trim (optional)

If you’ve used the ⅞” measurement if you put your HST units under your ruler they should be the finished size plus ½ inch for seam allowances. You may want to trim off the ears to reduce bulk in the final quilt.

If you cut your starting squares larger because you are planning to trim, place your ruler down on top of your HST unit with the diagonal mark on the ruler matching up with the seam.  Line up the ruler so that that diagonal line runs straight to the corner.  Trim any crooked edges off.  You should now have a nice precise seam at the corner and clean straight edges on two sides.

Flip the HST unit around and again line up the diagonal ruler line with the seam, making sure the seam aligns precisely at the corner. Match your clean edges up with the trimming measurement.  This is the finished size of the HST plus a ½ inch for the seam allowances.  Trim off the extra on both sides.

Making 8 HST units at a time

Making eight half square triangles at a time is just a matter of doing a bit more marking and sewing.  We start with a larger square that is the size needed to make four of the two-at-a-time units above.

Magic Eight HSTs Chart

Mark both diagonals

Both diagonals are marked, and seams are sewn ¼ inch from both sides of both of them.


I always start cutting by cutting through the centers.

After cutting they are just like the two at a time HST triangles above.  I cut them apart on the diagonal marked line and press as above.

Note that if you have a directional fabric, each set of squares creates two different types of squares.

Cut and sew cheat sheet for HSTs

I’ve created a cheat sheet for both of these techniques that you can download so you never have to look this up again! Just post it in your sewing room. By using the equation at the bottom you can find the size you need to cut for any size HST you want to make. Make sure to share this with a friend who needs it! If this link doesn’t work, please contact me and I will email you!

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