For pressing rounded corners, the holes in this hemmer let the steam through to your fabric, but you can also use them to mark the postion of the hemmer, and to pin the hemmer in place when you're ironing.   Made from silicone, the hemmer can take heat up to 230°C and won't distort with steam.  
 
The corners are 2, 3, 4 and 5cm radius.   The gauge marks either 0.5cm, 1cm and 1.5cm or on the imperial version, 1/4", 1/2" and 3/8".   There's a  corner ruler 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.