This is the fourth and last law of production scale. This law explains the situation that if we consider the variable factors of production as constant, then every additional unit of variable factor will increase the production as compared to the previous unit of variable factor of production.
But in this type of situation there will be point where proportion will start decreasing after reaching its maximum point, a time will come it will become even below then the point zero or will become negative.
In the table, we can clearly see the method on which this law works. In the table marginal productivity increases up to 3rd unit but in 4th unit it falls and starts decreasing, so that on 8th unit become equal to zero and on 9th unit marginal productivity become negative.