Yeah, my confusion lies in the effect of each centre. Imagine a tractor which is placed on a hydraulic machine which can tilt it. It topples when the line of action of weight is outside the wheel base. If CoG is what I understand it to be- the point at which the resultant moment due to weight is 0- then surely it is when the CoG is outside the wheel base when it topples, not when the CoM is outside the base? Am I misunderstanding the two terms or is the book being clumsy and using CoM and CoG interchangabely hence causing my confusion?