Updated: Oct 27, 2020
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Many patterns use half-square triangles. They are simple to make and very versatile. The challenge lies in getting them straight and even. An uneven hst will make for some messy looking lines and throw off your whole block.
1. cut the two squares you plan to use. I cut my squares 1/2" bigger than my desired half-square triangle (hst) size so that I can trim it to be perfect. These squares are 3.5" squares and will make (2) 3" half-square triangles once trimmed.
2. Make a mark on the back of one of your squares along the diagonal. I used a hera marker for this
Any non-permanent fabric marking tool will work. Markers should have ink that disappears in air or dissolves in water. Chalk or other white/yellow pencil marking tools will work best on a darker fabric. I like the hera marker because it makes an indentation that can be seen on all fabrics.
3. Lay your two squares with right sides facing each other so your marked back in facing up.
4. Line the edge of your machines 1/4" foot up with the center line and sew down one side. Turn your square and sew on the opposite side of the line.
5. Use your rotary cutter to cut along the center line to separate into two halves.
6. Press your two resulting half square triangles. I press with open seams but pressing to the darker fabric is done (and even preferred) by many.
Time to trim!
7. Line up the diagonal seam with the 45 degree angle on your ruler. These need to remain lined up during the whole trimming process.
Slide the ruler along this line until the left and bottom sides are lined up with the 3" vertical and horizontal lines on your ruler.
Trim the overhang from the top and right side
***If neither of your corners lines up well on both 3" lines, let your square go just outside the 3" lines (while still lined up with the 45degree line). Trim the top and right side. Then turn your square and line up the just trimmed corner on the 3" lines and 45 degree angle and trim top and right side.***
And there you have it! Two perfectly trimmed half-square triangles.