Sponge Hardness

Rubbers come with sponges with varying hardnesses. Harder sponges will give you more speed, but less spin. Softer sponges will let the ball sink in deeper, and generate more spin with less speed. Sponge hardness is measured in degrees:

Soft	up to 35°
Medium Soft	40°
Medium	42°
Hard	45°
Very Hard	47°+