How To Sew Half Square Triangles

Now that you know how to cut fabric and how to piece a 16-patch quilt block, you are ready for triangles!

When it comes to sewing, triangles are a little trickier than squares and rectangles, but still totally within a beginner’s skill level.  Follow along, and you’ll be able to sew triangles as well.  Or at least, half square triangles.  We’ll get to the other types of triangles in a future post.

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A half square triangle is exactly what it sounds like.  Draw a square.  Draw a line from one corner to another.  That’s a half square triangle, also called an HST.

Sounds easy, doesn’t it?!?!  Don’t stop reading here, there are a few more tips you’ll need to be successful.

Start with 2 squares, and use a pencil (or fabric pen) to draw a line from one corner to another, on the back side of one of your squares.

Use your acrylic ruler to draw your line, don’t eyeball it.  As you’ve learned in previous posts, precision is an important part of quilting.

For the Big Blue Quilt I’m working on, I need my blocks to be half color (blue or gray) and half white.  I marked the line on the white square, because it was easier to see the line on the lighter color.

I use a ruler for this step.  I’m good at quilting.  But I have terrible handwriting, and even drawing a straight line is beyond my skill level.  Seriously.  Even if you have awesome artistry skills, use a ruler anyway.  Precision makes all the difference when it comes to quilting.

Line up the two squares

and stitch a ¼ inch from the line you drew, repeating on both sides of the line.

Using the ruler and rotary cutter, cut along the pencil line.

Set the seam, and then press towards the darker fabric.

You’ve now got two HSTs!

There are an endless number of ways you can place your HSTs to make a block.  Please note, none of these “blocks” are sewn together.  What you see here is simply 4 half square triangles lined up next to each other and turned in a variety of ways.

For the quilt I am working on, I’m making a sort of triangle/arrow thingy.

Now, you may have noticed I didn’t mention any sizes in this tutorial.  This method for making HSTs will work no matter the size of your starting squares.  You just need two squares, and off you go.  As long as all your starting squares are the same size, you can make a whole quilt with half square triangles, and never need to know the math behind it.

But…

If your quilt looks like mine, a mix of squares and triangles, you have to involve math in the process.

Just like in the previous post about learning to piece, you have to take seam allowances into account.  Every time you sew two pieces of fabric together, some of measurement gets lost in the seam allowance.

The math for a half square triangle is the finished size + 7/8 inch.

There is some amount of a² + b² = c² in figuring out this math, but you don’t really need to know the details.  You just need to remember to add 7/8 inch to both height and length when cutting the squares to make your HSTs.

You now have all the tools you need to make a quilt top.  The remaining posts within the Basics Series are all about taking your quilt top and putting it together with batting and backing to make a completed quilt.