Quilting Basics : Half Square Triangles (HSTs)

Half square triangles are another important subunit in making quilt blocks and quilts.  A half square triangle block is a square made of two different fabrics, so that each fabric forms half the square joining along the diagonal.

This instructable shows how to construct a half square triangle block from 2 squares of fabric so that they are ready to be used in a quilt.  Calculating the correct size of starting square to end up with the desired finished size is also discussed.  After following this instructable a student should be able to construct half square triangle units as used in many traditional quilt blocks.

Step 1: Materials

2 squares of cotton fabric the same size

washable fabric marking pen or pencil
ruler

sewing machine
thread

or needle and thread if you hand piece (i.e. no sewing machine)

Iron
Ironing board

Scissors
Rotary cutter (not absolutely necessary, but makes cutting squares much faster)

My sewing instructions will all be for machine piecing, but the basic idea, cutting, pressing and layout are the same for hand piecing.

Step 2:

Take one of your squares and draw a line from one corner to the diagonally opposite corner using your washable pen & your ruler.

Put the 2 squares right sides together, matching the raw edges.

Sew 1/4 " away from each side of the marked line.

Step 3: Press the Seam, Cut the Blocks Apart and Press Again

Heat your iron up to cotton setting (3 spots), or a lower setting if you have used polyester thread. Most quilters recommend a dry iron for this i.e. no steam.

Put your pair of squares on the ironing board and then place your iron on the seam and press down on it for a few seconds without sliding it around.

Cut along the marked line using scissors or your rotary cutter & ruler, this will leave 1/4" seam allowance between the stitched line and the cut edge.

Put your pairs of triangles back on the ironing board with the darker fabric on top.  Now open the seam of the first pair (like a book) by folding the dark fabric over the seam and press it back in place, again try not to slide your iron about more than necessary as this can distort the fabric.  You want the fold of the fabric to be as close as possible to the stitches. Repeat for the second pair and you will have two squares each made of two fabrics.

You can now trim your block to be absolutely square if required.

Step 4: A (little) Bit of Maths

Your starting squares need to be 7/8" bigger than your finished (in the quilt) squares will be.

My starting squares were 3 7/8", and after sewing the diagonal seams they measured at 3 1/2".  This is correct.

When they are sewn to other quilt blocks with a 1/4" seam that extra 1/2" is taken up in the seams (1/4" on each side). Trust me, it works :-)