Summary The paper shows that the aggregate grading and compaction required for strong concrete produce an internal structural system which approximates to a series of lattices in two directions at right-angles. The diagonal members of the lattice are stiffer than the cross members, which represent the mortar in the “voids”, bonded to the diagonal members. The idealized system is analysed in terms of the relative stiffness of the lattice members, and equations are derived and curves plotted from which Poisson's ratio is related to stiffness ratios for various values of the ratio of axial compression to transverse tensile strength. The curves give a reasonable explanation of changes in Poisson's ratio and compressive strength due to changes in the secant elastic modulus of mortar due to creep. The paper also introduces a useful model technique by which the internal structural system of concrete can be represented. Ordinary draper's elastic is used to form a lattice having members of appropriate relative stiffness. The elastic is prestressed to prevent buckling. External compression acting on concrete specimens may be studied as external tension on the model, since changes of internal stresses and strains are sufficient to indicate internal distribution. Such models may be used to obtain the distribution of maximum tensile stresses in a concrete specimen under test, and hence the crack pattern and mode of failure for various shapes and external restraints.