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Sewing Half-Square Triangle Quilt Blocks from Fabric Squares — Tutorial

This is an easy method for making half-square triangle blocks from two fabric squares. Each set of two squares will make two mirror-image triangle blocks. This technique is the one to use if you want to make a few blocks each from a number of different fabrics.

Start by cutting two squares of the same size from two fabrics that look good together. I cut both of mine at once by laying both fabrics together on the cutting board.

Put the squares with right sides together and edges aligned. Use a ruler and a ballpoint pen to draw a diagonal line from corner to corner across the back of one of the fabrics:

Sew two seams 1/4″ from the center line on either side. Using a ¼” quilting foot makes this easy. The dotted red lines in the photo represent the seam lines:

Cut the blocks apart along the center line. You will get two triangular units that look like this:

Open the blocks and press the seam allowance to the darker side. Voila! You will end up with two mirror-image triangle blocks that look like this:

If you are making  a bunch of triangle blocks at once, you can speed up the sewing and save some thread by chain piecing the blocks. Chain piecing means that as you sew to the end of one block, you feed the next block under the presser foot  and sew right onto the next block without cutting the thread between blocks. This is what it looks like:

After you have sewn all the blocks, cut the thread between blocks to separate them.

Use a square ruler to square up the blocks and trim off the little tails at the corners.

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{ 6 comments… add one }
  • Pauline September 8, 2016, 4:07 pm

    Thank you for the excellent tutorial. I am new to patchwork any grateful for any help Hugs Pauline

    • Felicity Walker September 8, 2016, 4:39 pm

      Your kind words are much appreciated, Pauline! Please send us any questions that come up as you go along.

  • ana gomez October 2, 2016, 2:51 am

    gracias, me gusta esta clase clase de informacion. por que aprendo tecnicas de patchwork y quilting

  • arnetia November 26, 2016, 11:10 pm

    I am new to quilting and I would like to know, how big of square do I need to make a certain size finished block when making a half square triangle block.

    • cdmwriter November 27, 2016, 12:56 am

      Thanks for your question, Arnetia. The precise answer is that to make a half-square triangle block of 3″, for instance, you need to start with two squares that are each 7/8″ larger than the finished size of the triangle block. So to make a 3″ block, you would start with two squares each 3-7/8″. However, I find it very hard to measure and work with 7/8 of an inch, so I make my squares 1″ larger than the finished block. To have a 3″ finished half-square triangle block, I start with two 4″ squares, then trim the finished block down to the 3″ size after I sew it. This means you trim off a little more fabric, but I find it much easier to work with whole inches than 7/8″. Make it easy on yourself by getting a square ruler with a diagonal line you can align with the diagonal seam line in the block. This makes it much easier to square up a triangle block.

      Hope you could follow that answer. Once you make a few half-square triangle blocks, you will get the hang of it.

      Happy quilting!

  • arnetia November 27, 2016, 4:41 pm

    Thank you so much, your help is greatly appreciated. I agree, the one inch is much easier to remember than the 7/8th. I have the square ruler with the diagonal line and now I will just have to learn how to use it properly. Can’t wait to get started. Thanks again.

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