Most pulleys require an oval shaped karabiner to work properly. The OK is designed with just that in mind. It's oval shape also works week with belay devices as it doesn't matter which end you use!
|Petzl OK Screwgate Karabiner|
|Closed Gate - 24kN||19mm||75g|
|Open Gate - 7kN|
|Minor Axis - 10kN|