law of variable proportions

The law of variable proportions states that as a fixed factor of production is increased, holding all other factors of production constant, the marginal product of the fixed factor will eventually diminish. In other words, as more and more of a fixed factor is added, the output of that factor will eventually begin to decline; an example of this would be adding more laborers to a factory. Initially, the addition of more laborers would lead to a bigger output, however, after a certain point, the number of laborers would no longer have a substantial effect on the output of the factory.