When a production function assumes a fixed proportion between K and L, the isoquant takes ‘L’ shape. Such an isoquant implies zero substitutability between K and L. Instead, it assumes perfect complementarity between K and L. The perfect complementarity assumption implies that a given quantity of a commodity can be produced by one and only one combination of K and L and that the proportion of the inputs is fixed . It also implies that if the quantity of an input is increased and the quantity of the other input is hold constant there will be no change in output. The output can be increasing both the inputs proportionality.