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The graph on the left shows the relationship (green) between cumulative tonnes and the cut-off grade (COG). The higher the COG the less Cum Tonnes. The graph on the right introduces the secondary Y-axis which shows the relationship between Ave Grade above COG and COG. The higher the COG, the higher the Ave grade. So if you know what your product spec is (thus the Ave Grade), how do you determine the cumulative tonnes at that product spec...? Is it where the red curve intersects the green curve...? Sure looks like it... That's a common misconception. There is no relationship between the green and red curves as they relate to two different Y-axes. To determine the tonnage you first determine from the red curve where your COG is at the required Ave Grade, read from the X-axis. Then you switch over to the green curve and read the tonnage from the primary Y-axis on the left.

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