This is a unique ruler designed to help you with the half-square triangle. This ruler has all the math built into the ruler so you don't have to think about how much to add. All you have to know is the finished size of the block. You can also use this ruler to square up the blocks.
All you have to do is choose a finished size, layer two fabrics, cut, mark, and sew. Slots in the clear ruler to mark the sewing lines and cutting lines.