Quilting Basics : Half Square Triangles (HSTs)




About: Wife of one, mother of three & now a grandmother too! I enjoy making things and cooking edible things and eating them. aka on IRC as AstroMom

Half square triangles are another important subunit in making quilt blocks and quilts.  A half square triangle block is a square made of two different fabrics, so that each fabric forms half the square joining along the diagonal.

This instructable shows how to construct a half square triangle block from 2 squares of fabric so that they are ready to be used in a quilt.  Calculating the correct size of starting square to end up with the desired finished size is also discussed.  After following this instructable a student should be able to construct half square triangle units as used in many traditional quilt blocks.

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Step 1: Materials

2 squares of cotton fabric the same size

washable fabric marking pen or pencil

sewing machine

or needle and thread if you hand piece (i.e. no sewing machine)

Ironing board

Rotary cutter (not absolutely necessary, but makes cutting squares much faster)

My sewing instructions will all be for machine piecing, but the basic idea, cutting, pressing and layout are the same for hand piecing.

Step 2:

Take one of your squares and draw a line from one corner to the diagonally opposite corner using your washable pen & your ruler.

Put the 2 squares right sides together, matching the raw edges.

Sew 1/4 " away from each side of the marked line.

Step 3: Press the Seam, Cut the Blocks Apart and Press Again

Heat your iron up to cotton setting (3 spots), or a lower setting if you have used polyester thread. Most quilters recommend a dry iron for this i.e. no steam.

Put your pair of squares on the ironing board and then place your iron on the seam and press down on it for a few seconds without sliding it around.

Cut along the marked line using scissors or your rotary cutter & ruler, this will leave 1/4" seam allowance between the stitched line and the cut edge.

Put your pairs of triangles back on the ironing board with the darker fabric on top.  Now open the seam of the first pair (like a book) by folding the dark fabric over the seam and press it back in place, again try not to slide your iron about more than necessary as this can distort the fabric.  You want the fold of the fabric to be as close as possible to the stitches. Repeat for the second pair and you will have two squares each made of two fabrics.

You can now trim your block to be absolutely square if required.

Step 4: A (little) Bit of Maths

Your starting squares need to be 7/8" bigger than your finished (in the quilt) squares will be.

My starting squares were 3 7/8", and after sewing the diagonal seams they measured at 3 1/2".  This is correct.

When they are sewn to other quilt blocks with a 1/4" seam that extra 1/2" is taken up in the seams (1/4" on each side). Trust me, it works :-)

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    10 Discussions

    I've made about 6 or 7 quilts in the past 15 years, but all have been with blocks. Lately I've been looking at different patterns and they seemed alittle hard. With these instructions, not so much. What I thought would be a challenge now seems do-able . Thanks for giving great instructions and the confidence to do it. Would appreciate any other quilting tips you might have up your sleeves. Thanks: )

    1 reply

    Thank you for your kind comments. I do have some more instructables for basic units planned, I just need to find the time to take the photographs!


    7 years ago on Introduction

    This is an excellent write-up. And your learning objective is so concise! Looks good. Don't forget to show us the finished project (if there is one) in the comments or in a new Instructable.

    5 replies

    Reply 7 years ago on Introduction

    Thank you! Here are two photos showing the finished project a Spring/Easter Wall hanging for the lobby at work.  

    The block is from the Swoon pattern which simply uses squares, half square triangles and flying geese, so you could make this by following my already published Quilting Basics Instructables!

    The finished wall hanging measures 24" x 36"


    Reply 7 years ago on Introduction

    Beautiful work!

    You may want to edit this Ible and add these pictures as Wilgubeast suggested.

    Personally, I think the close-up picture would work perfectly as the Intro picture.

    Good Luck in the Challenge!!! ;-)


    Reply 7 years ago on Introduction

    Just a suggestion, but you might also want to add more keywords during the edit, such as:

    How to, make, sew, DIY, easy quilting, technique (etc).

    This will increase the number of visitors to your wonderful instructable. ;-)


    Reply 7 years ago on Introduction

    Thanks for the advice bajablue, I guess I tend not to be very good at blowing my own trumpet! I shall take a better detail picture showing the HST in situ and use that as you suggest.