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A composite material contains distinguishable constituent phases whose arrangement contributes to the bulk properties. In a fibre-reinforced example, strong fibres carry much of the axial load while a composite matrix holds them together and transmits shear stress between them. Carbon-fibre-reinforced polymer combines carbon fibres with a polymer composite matrix; glass fibres provide another reinforcement.

The constituent proportions alone do not determine the result. Aligned fibres produce anisotropy, whereas changing their orientations changes which directions are reinforced.^[ref] In an ideal long-fibre composite, the longitudinal rule of mixtures estimates the axial Young's modulus. A comparison of tensile testing along and across the fibres exposes the importance of geometry and fibre-matrix load transfer. The same idea of combining phase properties with geometry appears in effective conductivity, although the relevant constitutive equations differ.

A glass fibre is a thin strand of glass used as a reinforcement. When embedded in a composite matrix, many glass fibres can carry axial tension while the surrounding material maintains their positions and transfers load between them. The reinforcement is effective only if the interface transmits the required shear stress; an unbonded fibre can slide without carrying the expected load.

Continuous aligned glass fibres reinforce one direction strongly, whereas short fibres with varied orientations distribute reinforcement over more directions.^[ref] Even if the glass itself is approximately isotropic, a composite material containing aligned glass fibres is generally anisotropic. Comparing specimens with equal fibre volume fraction but different orientations separates the effects of constituent quantity and arrangement.