Beginning Quilting - Making Half Square Triangles

Making Half-Square Triangles (HSTs) is an essential skill in quilting. Half-square triangles are right-angle fabric pieces. The triangles are sewn together along their diagonal side, the long side. Thet are one of the top quilt block units in quilting. You can use HSTs in so many different ways. This method to cut and sew them is quick and accurate. Your points will always come out sharp if you follow these instructions. You’ll need two different fabrics, a light and a dark, your rotary cutter and non-slip ruler, a fabric marker, thread snips and an open-toe presser foot.

How to Cut Half-Square Triangles Accurately

Start by cutting squares. Make them at least 3/8" (and I do 1/2") larger than you want the finished half-square triangle (HST) to be. In other words, if you need the HSTs to be 2-1/2 inches for piecing the next step, cut the squares 2-7/8 to 3 inches square. In the example, I’ll sew 4-1/2-inch squares to get a 4-inch HST, which will then finish in a quilt at 3-1/2-inches.

Mark Before You Sew Half-Square Triangles

HST piecing is even more accurate when you mark the actual sewing lines instead of relying on sewing 1/4-inch away from a line.   To mark your sewing lines, place a ruler on the square, with the 1/4-inch line running diagonally from one corner to the opposite corner on your lighter color fabric.

Note the 1/4-inch line is on the point.

Draw a line at the edge of the ruler which will be 1/4-inch away from the center with a fabric marker.

Flip the square and mark a second line the same way on 1/4-inch away from the center.

Sew Before You Cut Half-Square Triangles

Pair the marked square with a second square right sides together. Most of the time this will be a focus fabric and the background fabric. Put the #29 Open Toe foot from the Madam Sew Ultimate Presser Foot set on your machine. This foot makes it easy to see your lines. Begin sewing on one of the lines. Sewing on the edge of the line nearest the center gives me a more accurate half-square triangle unit.

Chain Piece Your Squares

Chain piece by setting up all your squares, then running them one after another through the machine. As you approach the end of the first HST, place the next one in line with the first.

Continue sewing across the gap. Don’t worry, a few stitches without fabric won’t be a problem. It will create a little chain of thread between the pieces. Continue until you have all the squares sewn on one line.

The chain makes sewing the seam on the other side really easy as they are all together. Just turn the chain around and sew 1/4 inch away from the other side of the line.

When you get to the gap, just sew across like you did before and keep going. This makes the sewing go much faster than doing each one individually.

When you are done, you’ll have a chain of squares.

Cut the pieces apart from each other by snipping the little chain of thread between the squares. I love my Stork Scissors for this.

Cut and Press The Half-Square Triangles

Then cut down the center from corner to corner between the sewn lines to yield two HST units with quarter-inch seam allowances using your rotary cutter and non-slip ruler.

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Press toward the darker of the fabrics.

Square and Trim The Half-Square Triangles

Square your HST units to the correct size, placing the 45-degree line along the diagonal seam and trimming all four sides.

Trim two sides to the desired size, then turn the square to complete the trimming. Using a small ruler, like this one, you may need to turn several times. Just get the first cut accurate by aligning the 45-degree line on the sewn seam.

That's it! No need to be afraid of a lot of HSTs any longer. Quilts are so much more interesting with these angles as part of the design.

How to Use HSTs in Quilt Blocks

Now you can do all kinds of blocks! This pinwheel block is very popular, and is just four half square triangles.

Let’s look at sewing HSTs into blocks, there is one trick that you need to know. Sew this one nesting seams like you did in the first Beginning Quilting lesson using your Quarter Inch Foot to make your seam size accurate.

Then, press to the darker of the two sides, so you can nest the seams. Note that there is a quarter inch of seam allowance at the top of the points.

Now, pin your sections together, and note the little ‘X’ that has been created with the two previous seams.

Line up your sewing line to go precisely through that point. Using the quarter inch foot will make this easy.

Press the final block, and you have perfect points.

You can do flying geese this way too.

Or simply face the triangles opposite each other in rows.

Combine HSTs with the same size squares and you can get a Maple Leaf block, great for autumn-inspired quilts.

If you want more inspiration for blocks with half-square triangles, go to this blog post with 13 different layouts with HSTs in just two colors.

Next time you see a pattern with many HSTs, be confident, you can do it. In the coming weeks, we’ll do more basics and also begin to put things together for some fun designs, simple enough for beginners, yet fun enough for more advanced quilters to do as well.

Happy Quilting!


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