Half Square Triangles

Half square triangle units are very common in patchwork.  A square cut once diagonally yields two half square triangles.  Two half-squares sewn together to make up a square again is what I’m referring to as a Half Square Triangle unit or HST.  Here’s a visual:

The following describes my favorite way to make multiple HST units, 8 at a time.  Using this method, one avoids handling stretchy bias edges that can sometimes distort patchwork.

Measurements

Using the above colors as an example, I’d start by cutting 2 large squares, one of the purple fabric and one of the gold.  In this example, let’s say we wanted the HST unit to finish in the patchwork at 2″ square.  That means we’d need it unfinished at 2 1/2″ square to include a 1/4″ seam allowance on all sides.  In order to yield eight 2 1/2″ units, we’d need to start with both large squares measuring 6 1/2″ x 6 1/2″.

Here’s a reference chart:

Marking

On the back of the lighter (in this case gold) fabric, mark an “X” by drawing a line from corner to corner in each direction.  Like so:

Sewing

Pair the large 6 1/2″ yellow square (with markings) and 6 1/2″ purple square, right sides together and all edges aligned.  Sew 1/4″ away from both sides of both drawn lines.

Cutting

Make a series of cuts using a rotary mat, cutter and ruler.  Once you place the fabric on the mat, try not to move it between cuts. 

First cut: use a rotary ruler to align a center cut vertically through the square.  Again working with the above example, align the 3 1/4″ mark of a rotary ruler with the left hand edge of the square and cut along the ruler – this should intersect the drawn X through the center.

Second cut: again, without moving the fabric – align the ruler to intersect the square horizontally, and make one cut through the center.

Third & forth cuts: cut along each drawn line using a rotary ruler as a guide.

There will be 8 HST units.

Pressing

I almost always press the seam allowance toward the darker fabric.  Again, using the above example,  I would position the unit on my pressing surface with the (darker) purple fabric up, press the iron down on the seam (this is called setting the seam) and then open the unit, pressing the seam allowance toward the purple fabric.

Trimming

All units will be oversized and need to be trimmed to the exact dimensions.  This step is a bit tedious, but is also what ensures accuracy.  Use a rotary ruler with a 45 degree line and align the 45 degree line of the ruler with the seam in the unit.  For the first cuts, I trim very little fabric along the right and top of the unit, I don’t pay too much attention to the measurements at this time as long as the 45 degree line is aligned.  I just make sure I’m not trimming too much so that the unit will be less than 2 1/2″ square.  Then I rotate the unit so that the untrimmed sides are now along the right and top edges.  Now, I align the 2 1/2″ mark on my rotary ruler with the trimmed (left and bottom) edges AND make sure the 45 degree line is still aligned with the seam.  Then I make the final 2 cuts, one along the right and one along the top. 

There are different ways to trim.  Some quilters will trim before the unit is opened up and pressed, aligning the 45 degree line of a ruler along the seam and trimming through the two layers before pressing open.  Some people will trim after pressing but instead will trim to their desired measurement with the first cuts and then go back to trim the excess fabric (sometimes called dog ears) from the corners. 

Find the method you can live with and go for it!