In moving from point a to point b on TP curve in Figure 5.4, could be represented by the line (known as a chord ) between a and b (=TP/L). If the units of labour on the horizontal axis were to increase, for example numbering 1 to 120 rather than 1 to 12, then the geometric distance L falls and the length of the chord diminishes. As L approaches zero, a continuation of the chord effectively becomes a tangent to the TP curve at that point. Therefore, a tangent drawn to the TP curve at any point approximates to the value of MP at that level of output. (Note: this is the same geometric approach used in Chapter 3 when estimating the value of marginal revenue (MR) form a corresponding total revenue (TR) curve )