Rulers are easy to use when they are arched. 

ArchRuler

ArchRuler

ArchRuler is a ruler.
It's arched.

Pushing down in the middle, it becomes straight as a normal ruler.縲€You can steadily hold the paper with a little strength.縲€It utilizes the elasticity of the material.

Easy to pick up without using your nails due to the 1cm gap under the ruler. Easy to find when it's slipped into papers.

Good for a gift, attaching a line saying, 'an arched ruler.'

You can buy it from this store.
12 at random.