Bright Sisters: Half-Square Triangle Units

Two colors of fabric cut to 8" squares
Two 8″ squares

It all starts with two 8″ squares.

Bright Sisters has 320 half-square triangle (HST) units (3.5″ unfinished size) — not my favorite unit to cut and piece. Luckily for me, I discovered the Magic 8 Method (thank you, Karen Walker!) before I even bought the fabric for this quilt and I was able to plan my yardage and cutting diagrams to take advantage of this magical method. Visit Karen’s blog post or download Genius Hacks Every Quilter Should Know (thank you, Craftsy!) to get the full instructions and a few more excellent quilting tips. I’ll run through it quickly below. There’s also a Midnight Quilt Show episode where Angela Walters uses this method, if you want to see a video of it in action.

The thing that makes it a great approach for this quilt is that every matching set of HST units in Bright Sisters is a multiple of 8, so there was no waste and no units left over. It was meant to be!

You start by taking two 8″ squares, one in each color that you need for your HST unit, as shown above. Then you mark two diagonal lines across the wrong side of one of the squares.

An 8" square of blue fabric marked with two diagonal lines forming an X from corner to corner
Marked for sewing

Next, sew 1/4″ on either side of both lines, for a total of four seams.

Stitching next to the first marked diagonal line
Sewing the first seam
Sewing alongside the other side of the first marked diagonal line
Sewing the second seam
Stitching next to the second marked diagonal line
Sewing the third seam
Two 8" squares of fabric sewn diagonally with four seams
Ready to cut apart

When you have all four seams, you’re ready to cut the triangles apart. Simply cut vertically through the center where the seams cross, then without moving the fabric, cut horizontally.

Sewn 8" square being cut apart
Cutting the 8″ unit apart vertically and horizontally

When those two cuts are done, make two more cuts diagonally along the marked lines…

Rotary cutter, ruler, and partly-cut 8" unit
Cutting the piece apart diagonally

… and you’ll have eight half square triangles! It’s magic!

The larger unit cut into 8 HST units
Eight little HST units!

Or math. It could be math. But I prefer to think it’s magic.

Here I’m using a smaller cutting mat as a fake rotating mat, but after I did a couple of sets of these I broke down and ordered a real rotating mat. It made things easier.

Eight HST units
Cut apart and ready to press and trim

I made my original 8″ squares slightly oversize for the 3″ finished size HSTs I needed — Karen Walker’s tutorial has directions for how to size your starting squares, and she recommends making them a little bigger than you need — so I still have to press and trim each one. (As we will discover in a later blog post, it’s a really good thing I did make them oversize…)

Blue and green HST unit
Pressed and ready to trim

I have a nifty little June Tailor ruler that I love for this kind of trimming. You line up the unit so the diagonal seam matches the marking on the ruler, make sure the four edges extend past the horizontal marks and vertical cutting channels for the size you need, and trim the first two sides:

Half square triangle unit and trimming ruler
Trimming the first two sides

Then you turn the unit 90 degrees, line up the diagonal seam and the two trimmed edges, and cut again:

Half square triangle unit and trimming ruler
Trimming the last two sides

It made quick work of trimming 320 HST units!

Blue and green HST unit trimmed to size
One trimmed unit!

One down, 319 to go! What’s your favorite method for making HST units? Any good tools you can recommend?


What do you think?