This is a great technique for making Half Square Triangles that eliminates the need to directly manipulate the stretchy bias of the triangle. It utilises two easy-to-cut squares.
You will be given instructions to cut pairs of squares, as this technique produces two Half Square Triangles. I usually give generous measurements so you have lots of wiggle room when trimming the units to exact size (for instance if you need a 1 7/8″ square, I will tell you to cut an even 2″ x 2″).
The rule for working out the size of square to cut to make Half-Square Triangles:
Add 7/8″ to the required finished size.
On the back of the lightest fabric (or the one most likely to show a pencil mark), draw a pencil line, diagonally from corner to corner. Repeat for every square you need in this fabric. No need to draw on the dark square; you only need a line on one square in the pair.
Stack a pair of light and dark squares, right sides together. Sew a scant ¼” seam either side of the diagonal pencil line.
Hint: some books don’t give you much wiggle room when it comes to trimming the Half-Square Triangles. In this case I often make my ‘scant’ seam even ‘scantier’; but, usually don’t let it get much smaller than 1/8″.
You will end up with something like this:
Now, cut along the diagonal line.
First, press the seam together, to ‘set the seam’; then, press towards the darkest fabric.
To trim the block to the exact size: firstly, line up the diagonal (the 45° angle) on your ruler with the seam.
Then, carefully trim your block into sheer perfection.
The result: a perfect Half-Square Triangle.
If you have lots of Half-Square Triangle to produce in the same colourway, an extension of this method is to sew two strips with a pair of zig-zag lines. You are basically sewing many units at once.
For instance, if you have to make ten Half-Square Triangle units, rather than cut five squares of both fabrics, you can cut a strip, its length equal to the five squares laid side by side. Draw a line where each square begins and ends on the lightest fabric. Use the unfinished size; so, if you need a 1″ x 1″ finished square, use the rule above to determine that each square needs to be at least 1 7/8″ x 1 7/8″. Rule these lines on the wrong side of the fabric:
Then draw the diagonals in a zig-zag fashion along the strip:
Sew a scant ¼” seam on either side of the drawn line:
Cut your units apart, on the vertical lines and the diagonals. You know the rest!
I’m not entirely sure this method works any more quickly than cutting individual squares. I’ll give you cutting instructions for individual squares, and leave it to you to choose the method that suits you best!