Squaring up Half-Square Triangles

It has been so fun to already see blocks popping up in the Virtual Quilting Bee Flickr group. One of my favorite things about quilting is seeing the variety of colors and fabric choices people use. 
I’ve decided to make my own version and spent the morning pulling fabrics from my stash in a new palette I’ve never tried. It’s a bit of a break-away from my traditional primary-color palette that I use most often. I’m kind of excited. Most of the fabrics are bits and pieces from favorite fabric collections from days of yore like Denyse Schmidt’s Hope Valley, and a few of her collections from JoAnn’s, Joel Dewberry’s Deer Valley plus I’ve thrown in a bunch of the recent Bike Path Dots by Alison Glass that I’ve been itching to create with.  I’m excite to play with this stack. 
When making my block, I remembered that a couple of people have asked me for a quick tutorial on squaring up half-square triangles. So here’s a quick explanation: Half-square triangle (HST) blocks are one of the most versatile blocks in quilt-making. They are quick and easy to make as Sherri showed by drawing a diagonal line on the back of one of the squares, matching the two prints right-sides together, and then sewing down either side of the line with a scant 1/4″ seam allowance.
Then cut on the drawn diagonal line and press both blocks open and voila: two identical half-square triangle blocks! As Sherri mentioned in her tutorial, you are going to want to square-up these particular HST to 2 1/2 inches – that means trim them to the right size and make the sides straight – or “square”.  I personally like making my HST’s slightly large and squaring- up the block when I’m done because it makes the blocks so much neater to work with.
Even with the most careful of sewing, when you sew on the diagonal you are sewing on the bias. Drawing that line and sewing before you cut, will help keep the diagonal edges from stretching but there will still be some misshapen edges on those blocks. Squaring-up will make things nice and neat, not to mention the right size, in many cases. 

One of my favorite tools for squaring up HST blocks is a square ruler with a diagonal line. (There are lots of nifty tools for such purposes.) You don’t need to have this – it’s definitely possible to square-up with any ruler – but if your quilting budget allows for one of these, over time it’s a great investment.

When squaring-up a pieced block, you want to be careful to keep that diagonal seam running from corner to corner. If your ruler has a diagonal line on it place that line right over the seam. This will keep your seam centered and your block square. If your ruler does not have the diagonal line, place the corner point of the ruler over the seam and make sure the point you are measuring to (in this case the 2 1/2″ point) also on the diagonal seam at the other end.

First trim two sides, then rotate the ruler so that the 2 1/2″ lines (in this case) are on the newly squared-up edges and square-off the other two edges. I know it’s a little more tedious to square off all 4 sides, but it definitely makes that block neat with straight edges on all four sides.

If the block has fairly square sides before sewing up, you could place the 2 1/2″ lines directly on the left and bottom edges (right and bottom if you’re left handed) and just square-off the right and top edges. Your call. Make sure you trim those little ‘tags’ in the corner though, so you won’t have a lot of bulk when assembling your block.

Post Edit: I have since found a new favorite short-cut method for squaring-up HST’s using a specialty ruler.

Hope that helps!

And just a reminder – I’m answering questions as much as possible in the comment sections of each post.  Let me know if I’ve missed something. Thanks again for playing along!


  1. says

    amy this is an awesome post! I just finished trimming a boat load of HST&#39;s last night! I wondered how everyone else did it, and I&#39;m also glad to know that everyone&#39;s edges are sometimes a little wonky … good point that you&#39;re sewing on the bias when you go from corner to corner. <br /><br />

  2. says

    Amy,<br /><br />Thank you so much for a wonderful post. I am enjoying the viirtual quilting bee.<br /><br /><br />Have a great night.<br /><br />Linda

  3. says

    Amy!!! Thank you so much for this tutorial! I am going to give this a try and see if I can make some HST&#39;s that are not wonky!! You are terrific!! :)

  4. says

    Great information, but a quick question. When you say you make your HST&#39;s slightly larger, do you add 1/4&quot; to the sides of the squares? 1/2&quot;?

  5. says

    What a great fabric collection!!! Please come to Philly and help me to pull mine together!!!<br /><br /><br /><br /><br />Courtenay<br /><br />p.s. Great explanation of &#39;squaring up&#39; these triangles.

  6. says

    Amy, this might be a dumb question, but I&#39;ve always wondered, so I might as well ask :)<br /><br />When you square up the blocks, are you trimming away your backstitching? Should you backstitch a little more that usual so as not to trim so much off? Or, if you are trimming it, is it not a big deal since the blocks will be sewn to another square anyway?<br /><br />If you have time to answer,

    • says

      Great question, Rachel! I generally don&#39;t backstitch at all when I&#39;m piecing. (The only exception is when I am sewing Y-seams, such as when I machine-piece hexagons.)<br /><br />Other than that, you don&#39;t need to. As you said, the seams and blocks intersect so much, that they stay stable. Saves a lot of time too and makes it much easier to chain stitch (sew pieces together one after


Leave a Reply

Your email address will not be published. Required fields are marked *