This is because, if the volume is to remain constant, the changes in the heights of the various faces must be such that when multiplied by their surface areas the sum is zero. If there were only two surfaces with appreciable area, as in a pancake-like crystal, then O 1 / O 2 = − − δ δ ( h 1 ) V c / δ δ ( h 2 ) V c . In the pancake instance, O 1 = O 2 on premise. Then by the condition, δ δ ( h 1 ) V c = − − δ δ ( h 2 ) V c . This is in agreement with a simple geometric argument considering the pancake to be a cylinder with very small aspect ratio. The general result is taken here without proof. This result imposes that the remaining sum also equal 0,