The behaviour of marginal product and total product as more and more units of only one input are employed while keeping other inputs as constant can be understood with the help of following diagram:  [Ref. image] From the figure, we can analyse that initially as more and more units of one input (say, labour) are employed, the TP curve increases at an increasing rate till point I. After point I, the TP curve rises at a decreasing rate till the point B. The point I is also known as point of inflexion. This is because passing through the point I, the curvature of the TP curve changes from convex to concave. With successive rise in the labour units, the TP curve continues to rise and attains its maximum point B. Beyond this point, the TP curve starts falling when more units of labour are employed. On the other hand, MP initially increases at an increasing rate and attains its maximum point at M (point of inflexion). After point M, MP curve continues to fall and becomes zero at N. Finally. after point N, MP curve continues to fall and becomes negative.  So, we can summarize:  a. When TP increases at an increasing rate, MP also increases at an increasing rate.  b. When TP increases at decreasing rate, MP attains its maximum point and starts falling.  c. When TP reaches its maximum point, MP becomes zero.  d. When TP starts falling, MP becomes negative and continues to fall.