The downward-sloping section of the VMPL curve defines demand for labour (L) by a profit-maximizing firm. It slopes downward because of the law of diminishing marginal returns, which implies that the marginal product of an input (in this case, L) declines as more of that input is used. Therefore, the value of the VMPL also declines as more of the labour (L) input is used (in the production process)