A half-square triangle unit (HST) is a patchwork square made up of two right triangles. Each triangle takes up exactly half of the square's interior. No matter what you call them, you'll find that HST units are among the most popular types of patchwork found in quilts, so it's a good idea to learn more than one method for their construction. You can sew two right triangles together to create a triangle square, but individual triangles are stretchy on their long, bias edges. Why bother when quick piecing methods eliminate the need to fool with the bias?
If all you need is two finished half-square triangles, the HST unit assembly method is the right technique. It's easy and fast, and perfect for scrap quilts. The magic 8 method is very similar, but it doubles-up the results by sewing the equivalent of four paired squares simultaneously. With the magic 8, making enough HSTs for a quilt like the Hunter's Star is easy peasy.
Years ago, many of us sewed HST units by securing fabric to a grid drawn or printed on paper, following the sewing lines before cutting the patches apart. The magic 8 method is a simplified version of that technique. We'll draw only a small section of the grid, and make lines directly on the fabric, not paper or another material.
Read through all the instructions to get an overview of making magic eight HSTs. Use any type of marker that won't bleed into (or rub off onto) fabric, something with a sharp point if possible.
Grids Drawn grids are still popular and now available commercially, although most are made from long strips rather than a large square or rectangle. Square grids can work nicely for small HST units, but they become awkward when sewing larger patchwork. When marking a grid directly onto the fabric, the larger the grid, the higher the likelihood of error.
Equipment / Tools
- Marking pens or pencil
- Quilting rulers
- Rotary cutter and mat
- Sewing machine, set up to stitch
- Iron and ironing board
- Cotton fabric scraps for practicing
Your pattern should tell you the finished size of the square HST units. The formula for calculating how big to cut your two contrasting fabric squares is:
- Multiply the finished size by 2.
- Add 1-3/4 inches.
Cut two (contrasting) squares of fabric to match the calculated dimension. For your practice HST, let's make 3 x 3 inch HST units.
- 3 inches x 2 = 6 inches
- 6 inches + 1-3/4 inches = 7-3/4 inches
Your contrasting squares would each be 7-3/4 x 7-3/4 inches.
Drawing The Grid
Square up your fabric and rotary cut your squares. Place the squares on top of each other, right sides together, lining up edges.
Using the marking pen and ruler, draw two diagonal lines on the top squares, corner to corner. I tend to mark the lightest square, but keep white markers on hand to use when both squares are relatively dark.
Secure the squares together with pins so the fabrics don't shift.
Sew a 1/4 inch seam on each side of each diagonal line. If you don't have a quarter-inch presser foot, mark the lines before sewing.
Press the sewn square to set the seams, a step that removes little puckers and makes it easier for fabrics to lie flat later.
To separate the square into eight units you will be making four cuts: a vertical, a horizontal, and two diagonal. As you make the cuts, don't rotate the fabric or your pieces may shift position! Instead, rotate the mat or walk around it.
- Vertical Use the center diamond shape created by the seams, the original marked lines, the lines on the mat, and the sides of the shape itself to guide you as you line up your ruler. With the rotary cutter, slice through both layers of the sewn square at its vertical midpoint.
- Horizontal Use the same technique to cut your halved square into quarters.
- Diagonals Finally, cut each of the remaining sections apart on the original drawn diagonal lines.
You will know have eight triangles of one fabric stitches diagonally to eight triangles of a second fabric. Now it's time to press.
- Take the eight unopened HST units to your ironing board. With the fabric the seams will be pressed towards facing up, press the unopened HSTs to set the seams.
- For each unit, flip the top fabric of back and use your iron to (carefully) press the unit open.
You now have eight half-square triangle squares!
The eight HSTs in the photo have their little "doglegs" attached—tiny triangles that extend past the seam when patchwork is pressed. Clip them off before use.
Congratulations, you just learned the magic 8 method!