Friday, September 19, 2014

Making Half-Square Triangle (HST) Units

This tutorial shows how to make HST units using the stacked squares method. In this method, you cut one square from each of two fabrics to create two, identical HST units.

Determine the finished size of the HST unit that you want to make. To that measurement, add 7/8”. Using this sum, cut one square from each of your fabrics. In this example, I’m making a HST unit that finishes at 3”, so I’m cutting my two squares at 3-7/8”.

Since we’ll be dealing with a bias seam, I recommend spraying your fabrics with starch before cutting them.

Time saving tip: Layer your fabrics right-sides together when you’re cutting your squares. This way, both fabrics are cut to exactly the same size, and you don’t have to spend time matching edges before you sew.

Here, I’ve layered my fabrics, then cut them into strips that are 3-7/8” wide. Now I’m ready to sub cut my strips into squares.

NOTE: When I measure my fabric, I’m making sure that the edge of my fabric is hitting the outside edge of my measurement line.

Now we’re ready to sew!


Draw a line diagonally across your lighter fabric.

Sew a scant 1/4” away from this line. Note in the right-hand photo, that my drawn (pink) line is directly under the 4-3/4” ruler mark, but my needle is positioned just to the left if the 4-1/2” mark (blue dashed line). My seam is just a little less than 1/4" wide.

(Alternatively, I could have my drawn line just under the 4-1/4” ruler marking, and then sew with my needle just to the right of the 4-1/2” marking.)

You might be able to see this better after the stitching is done. My drawn line is under the 1/2” mark, and my stitching lines are just inside the 1/4” and 3/4” marks.
My seams are a scant 1/4” wide.

Now cut between your stitching lines to separate the two HST units.


After setting my seam, I open my fabrics part way, and then I very gently roll my fingertips over the seam to start pressing it open. After finger pressing one section, I’ll lift my fingers and move them down to gently finger press the rest of the seam. Only then do I press the seam with my iron.

Do not use steam when pressing this seam, and make sure that you are pressing the seam (using an up & down motion with the iron), not ironing it (rubbing back & forth)!

Squaring Up:

I know you want to get on with the rest of your sewing, but don’t skip this important step!

Square up (trim) your HST unit so that it measures your unit’s finished size + 1/2”. In this case, my finished size is 3”, so I’ll trim these at 3-1/2”

Use a ruler with a 45° marking on it. Place the 45° marking on top of the seam line. Make sure that your unit extends just past the desired measurement, and then trim the right and top sides of the unit.

Now turn the unit around 180°. Place the 45° line on the ruler back on top of your seam line. Make sure that the trimmed left and bottom edges come all the way to the outside edges of your ruler markings, and then trim the right and top edges.

You might not trim away much
when you square up your HST unit,
but that little bit can make a big difference!

Sewing with HSTs

One final note to remember: Whenever you sew an HST unit to another unit, the diagonal seam will enter the new seam 1/4" before the raw edge of the new unit. When you square up the new unit, be sure you don't trim off your seam allowance outside the triangle points!