What would happen if I use a rope less than 12mm for hanging?

[purplesky9]

I have a rope that is 10mm. Will this break?

 

[Hollowman]

Test it. I won't tell you how. There's probably some threads about it.

 

[Worndown]

The thinner the rope, the more uncomfortable it will be. 12mm is the "sweet spot" when it comes to comfort.

 

[purplesky9]

But it should be strong enough

 

[Worndown]

It should be, but not always. Some tope is strictly ornamental.
Test it. Tie it to something, grab it and see if it holds you.