How to Make a Stack of Half-Square Triangle (HST)

If you’re interested in diving into designing your own quilt, you might start with quilts using half-square triangles, also known as HSTs for short.  HSTs are squares sewn with two triangles, just like the name implies.  Interestingly, Mark Jason Dominus, a computer programmer, determined all 72 possibilities for arranging four HSTs into a pattern to impress his quilter girlfriend in 1994.  She later became his wife!  His results resulted in a fascinating graphic.

HST Possibilities

I’ve taken the time to create a Pinterest Board devoted to HST quilts in an effort to get our collective design juices flowing.

You may notice that many of the HST designs employ squares made up of one half light triangle and a darker other half.  Of course, others stir up the design pot and piece one or both halves.  Some quilts use whole squares surrounded by HSTs.

Ingredients for a half square triangle quilt

I decided to play with a Moda Charm Pack™ I bought last year, “Gypsy Girl” by Lily Ashbury.  The charm pack came with 42 5″ squares.  I decided to make HSTs that were half solid in cream and half one of the Gypsy Girl print.  Therefore, I cut five, 5″ strips (width of fabric or WOF) of the cream colored Bella Solid you see in the picture above.  I then subcut each of those 5″ strips into eight 5″ squares.

Measuring for the HST quiltCut the squares down the middle

I then marked my sewing lines on the back (wrong) side of the solid fabric.  We’re going to make two HSTs at a time.  In the first picture (on the left) you can see how to do this with a special 1/4″ seam adder ruler by Fons & Porter.  If you use the seam adder ruler, you simply use a pencil and mark it down either side of the ruler.  *Be careful to check to see that the ruler has not shifted.*

You can also use a regular quilting ruler.  You line up the 1/4″ line with both the points of the rectangle.  You draw a line, then flip it over and draw another line.  Some people advocate drawing one line down the middle and then sewing a 1/4″ from each side of the line.   I have not had accurate results with that method; therefore, I take the little extra time to draw both lines.

Roping in my child to mark the HST quilt

As a “time saver” I talked my 5-year-old into working on this project with me, and had her mark the sew lines.  (Of course, it did not really save time, but any time I get to sew with my little one and teach her about the quilting process is time well spent!)

Placing the marked square on top of the printed one

The next step is to place the pre-cut 5″ square, facing up, next to your sewing machine.  Line up the solid square, marked side up, on top of the patterned one.  As you can see, I talked my little one into doing this step for me, too!

Sewing on the marked lines

Using my edge stitching foot (see how it has a bar in the middle that allows you to follow lines?), I simply sewed along the marked line.

Chain piecing the HST quilt!

To save thread and time, I chain stitched all 42 squares.  I removed the string of squares from my machine.  Starting with the square that I ended with, I sewed the other line in a chain.  Guess who cut my squares apart?

Stacks of sewn squares

This left us with a stack of squares with two lines sewn.

Cut the squares down the middle

The next step is to line up the square so that you cut down the middle.  This is also the perfect time to determine if your helper drew the right lines or if your sewing was accurate.  If it was not, unsew the offending stitching line, and start over.

HST square blocks

After cutting the squares in half, you will have a stack of 84 triangle units.  At this point, you will need to press and square up these units.  I found one quilter, Amy Smart of “Diary of a Quilter,” who raves about a special Quilt In A Day™ Ruler as a means of squaring up the HST.  This method squares the block before opening and pressing the block.  I have not tried this method, and wonder if it works as well as the old-fashioned way of squaring the block.  Nonetheless, I’ve put this ruler on my Amazon wish list in hopes that I’ll get one for Mother’s Day!

HST square blocks after pressing the seam open

After setting the seam and pressing my seams open, I have a stack of 84 HST units.

Time to Square up the square units

Time to square up the units.  I have to square up my units to 4.5″ – my piecing is just not that perfect.  Without squaring these units, I would be pulling out my hair later when trying to piece these units together.  I happen to have a 4.5″ ruler from another project.  This makes the process easier, but it is not necessary to have the perfectly sized ruler.

Nest two squares together

To save a little time, I squared up two HSTs at a time by nesting them together carefully.


I lined up the 45º line on the ruler with the seam line.  Because I’m right handed, I then cut the right side and the top side.  In the picture above, these squares were pretty accurate.  Not all of my units looked this good!

Turn the rectangle 180 degrees and cut the right and top again.


I then turned the square 180º to trim the remaining two sides, which are now on the right and top.

Final stacks of delicious HSTs Oops!  Some piecing problems that need fixing.

The picture on the left shows my delicious stacks of HSTs.  The one on the right shows two HSTs that will need to be unseen and redone!  No one is perfect, but isn’t that part of the fun?  Learning how to avoid mistakes you have made on each project?

Here is what I made with this particular stack:


Stay tuned for the visual possibilities of my next stack of HSTs!  Happy Crafting!

Leave a Reply