I enjoy using the method that incorporates the 7/8" measurement which produces 2 of the necessary half square triangles without waste of my fabric. But I have students come to me asking how they can get around that measurement, it seems for some that is an unfamiliar measurement and they are uncomfortable and unsure of the results they will receive if they attempt to cut a square using a measurement that incorporates a 7/8" mark on the ruler.
I would like to go over the steps I use in attaining a perfect half square triangle without using the 7/8" measurement.
Let's say we want to make a 3 1/2" half square triangle.
CUT: 2 coordinating 4" squares
Place your lightest color square on a sandpaper board for marking. The board discourages slipping as you are trying to mark upon the fabric. I like to use Omnigrid's Marking Ruler Trio Set for marking the sewing line for my half square triangles. You place the center black line of the ruler on the diagonal of your fabric square and mark on both sides of the ruler
The prepared square is now ready to sew
Sandwich the other square to your marked square right sides together. I like to use an open toe foot when I sew these triangles as it makes the drawn lines more visible to me. I place my needle about a thread width to the inside channel made by the two drawn lines. This will help to compensate for thread and pressing which makes the block a little smaller than intended.
Sew down the length of both drawn lines
Cut 1/4" from one of the sewn lines to create 2 half square triangles.
Press.....don't worry about the dog ears right now
Place the 45 degree angle line on your ruler on the sewn diagonal line of your block. Opposite your cutting sides line your ruler up so it is just a little over your target of 3 1/2"in both directions.
Turn the block around and repeat cutting the other two sides but now the two sides will be on the 3 1/2" marks.
Your block should now measure a perfect 3 1/2". To the right of this photo is the dog ears and small amount I cut off to true up my perfect 3 1/2" half square triangle.
Here is one of the perfect squares using a 4" square instead of 3 7/8" square in the beginning.
I hope this will make half square triangles more fun for you if you are uncomfortable using the unfamiliar ruler measurements. This also works for 3/8" (use the 1/2" mark) and 5/8" (use the 3/4" mark).
Hope you enjoy this method and have fun with half square triangles.