This is a block/unit called by several names. Mary’s Triangle, Magic Triangle, Brave New World, Shaded Four Patch, etc. I’m calling them Magic Triangles because it’s magic the way this works.
There is a very good video demonstration on how to put this block together on YouTube. The following chart just gives you the finished size of the block and then what size pieces you need to cut.
The formula for this is as follows:
Determine what your finished size for the block will be.
1. The corner square is half of this measurement plus 1/2 inch.
2. The small rectangle short side (width) measures the same as unit in step 1. The long side (length) is that measurement plus 1 inch.
3. The large rectangle width is the measurement from step 1 multiplied by 2 minus 1/2-inch seam allowance. The length is that measurement plus 1 inch.
So, if your block finishes at 4″, your small corner block would be cut at 2.5″, the small rectangle would be cut 2.5″ x 3.5″, and the large rectangle would be cut 4.5″ x 5″.
You can download a PDF chart with finished sizes ranging from 1.5″ to 18″ by clicking on the button below.