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Tricky Triangles

By Barb Bergquist

You may have heard that triangles are tricky, but not these!  The tricks here will make these triangles easy to make every time.  Read on and learn how to construct half square and quarter square triangles!


Do you know the easy way to make half square triangles (abbreviated HST)?  Let me give you a lesson on the process.  First, let’s talk about four numbers.  First, what size do you want the HST to be?  The finished size is the size that it is incorporated into your pieced item.  The size with seam allowances will be ½” larger than the finished size.  So let’s pause here while I’m demonstrate with real numbers to help you see the relationship between these.  If you need a 3” finished HST, then you will be making a 3½” HST with seam allowances. 

HST Finished SizeHST Finished Size with Seam Allowances

Now comes the fun part.  The HST is made from two different fabrics.  You will start with squares of each fabric that are ⅞” larger than the HST size with seam allowances.  So in our example, 3½” + ⅞” = 4⅜” hence you will cut squares from each fabric that are 4 3/8”.  When you finish this process, you will have exactly 3½" HST, assuming your cuts and seam allowances are accurate.

Size to Cut for HST

If you don’t want to trust that everything will turn out perfect and would rather trim the sewn HST to the size with seam allowances, then you will add 1⅛” to the HST size with seam allowances.  Again following our example from before, you will add 3½" + 1⅛” to get a square size of 4⅝” that you will cut.  Once you’ve finished sewing, you will trim the resulting HSTs to your desired size, in this case, 3½".

Size to cut to trim HST

Once you have the squares cut from each of the two fabrics, you will draw a diagonal line from one corner to the opposite corner on one of the fabrics.  The next step is to stack the fabrics so they line up perfectly on top of each other.  If you wish, you can pin the two together at either end of the diagonal line to ensure that they do not slip while sewing.

Cut Size with Diagonal Line for HSTs

Now sew ¼" from each side of the diagonal line.  When you are finished, you will have ½" between the two sewn lines.  Here’s where the magic happens – cut between the two sewn lines, on the diagonal line.  The result – two half square triangles.  It’s like magic!  Try it out; I’m sure you’ll love this trick.

Cut Size with Sewn Lines for HSTs
Cut for HSTs
Finished HSTFinished HST


QST is Quarter Square Triangle, but you’ve probably already guessed it.  I’m going to demonstrate with two fabrics, but you can also use four fabrics when making QST.  The one things to be aware of when using this method with four fabrics, is that half of the QSTs will be the mirror image of the other QSTs.  If you want all QSTs to be identical, then either you make twice the number of squares which will give you half left over, or you don’t use this method.  If you are doing two projects, then no leftovers, right?

So let’s start with the math again.  What is the finished size of the square?  The finished size, as I explained previously, is the size it will be once pieced in your quilt project.  The size with seam allowances will be ½” larger all around than the finished size.  Using real number, I’m going to pretend that I need a 4” finished QST, so it is 4½” with seam allowances.

Finished Size for QSTFinished Size for QST with Seam Allowances

To work with the exact size, you will add 1¼” to the finished size of the QST.  (Note that this is a little different from the HST.  Don’t confuse the two!)  For a 4” finished square, I will start with 5¼” squares of fabrics.  If you want to trim it to fit the finished size when the process is complete, add 1½” to the desired finished size hence cutting 5½” squares.

Cut Size for QSTCut Size for QST to be trimmed

The first step you already know.  You are going to make two HSTs.  Draw the diagonal line, sew ¼” out from the diagonal line on both sides, cut them down the middle following the diagonal line and press the HSTs open.  You are half done!

Now this gets a little tricky, but not a lot.  Draw a diagonal line on one of the HSTs so that the diagonal line crosses the seam.  Next, place the two HSTs right sides together with opposite fabrics against each other.  Nest the seams of the two squares (I suggest you pin them), and again sew ¼” out from the diagonal line on both sides of the line.  Yes, you guessed it – now you cut on the diagonal line and press the squares open.

HST with Diagonal Line Drawn for QST
Sewn HSTs for QST
Finished QSTFinished QST

And voila!  You have a QST.  If needed, trim the squares to the size with seam allowances.  In my example, I would be trimming mine to 4½” squares.

That is all there is to it.  Simple, right?!

Happy quilting!


Barb Bergquist along with husband, Ron, own A Block Away Quilt Shop.  A dedicated quilter with more than 25 years of experience, she is now actively sharing her love of quilting through the work in her shop.

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Phone: (956) 638-1961
Location: A Block Away Quilt Shop
                 2706 N. 10th Street
                 McAllen, TX 78501
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