An isoquant is a curve in the K-L plane, which represents different combination of inputs, K and L, that would produce the same outpu. Thus, along a isoquant the output remains constant while the inputs vary.

The slope of the isowuant is given by the ratio of the marginal productivity of labour and the marginal productivity of capital, known as the MRTS.

MRTS = MPl/ MPk

The red curve represents an isoquant. It can be noticed, that at the point a and b, the slope of the isoquant with their respective axes becomes 0.Before the point a and b, the slope of the isoquant is negative, Beyond the point a and b. the slope of the isoquant becomes positive. In between the point a and b, the factor of production, K and L are substitutes, i.e a producer can increase a factor by decreasing the other. However, beyong the point a nad b, there lies a positive relatonship between the factors, and to increase the quantity of output, both the factors must be increased. This would be irrational as correcponding to each point beyong a and b, there lies a point where the producer can produce the same output by using less of one input and more if the other, rather than using more of both the inputs,

Thus, a rational producer must choose to produce between the area a and b, where the slope of the isoquant is negative.

The diagram below, depicts an isoquant.

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