Rulers are easy to use when they are arched.
This is a ruler.
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.'