For drafting rounded corners on pockets, bags, shirt tails etc.  The corners are 2,3,4 and 5cm radius on the outer edge.  The width of the ruler is 1.5cm or 5/8" so you can draft your seam allowance at the same time.   There's a  silicone hemmer exactly the same shape, and a deal if you want to buy them both together.
 
If you're buying from outside the UK - please buy through Etsy to have the correct tax applied. This should save you paying any local handling charge in addition to any tax.