From the 4 Wheel Parts website...

"Inventive lug design earns these Pro Comp tires a Severe Weather rating, while advanced tread patterns ensure even wear and maximum durability. The tread is designed with an elliptical siping pattern and interlocking shouldered lugs that provide extra bite and gripping power in snow, dirt, and sand."

These tires are very good, and I will be going back to them when my Xterrains wear down. Only in 33" rather than 31".