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°+