The zones on this plot represent the different 'classes' of materials: metals, polymers, composites, etc.. The guide lines show constant values of ratios of (modulus/density), (modulus/ density²) and (modulus/density³). These quantities are used in selecting materials for different tasks. Materials on the same line will offer the same performance based on these criteria. Selection can then be made on the basis of other favorable attributes, such as corrosion resistance or manufacturing.

From: Ashby, "Materials Selection in Mechanical Design," Pergamon (1993)