A physical picture of a conjugated chain as a collection of almost planar segments, separated by large angular breaks arises from a microscopic model which includes conjugation and steric interactions. The conjugation part of the standard phenomenological Hamiltonian for torsional motion is also derived from the model. We obtain a probability distribution of the length of segments between those breaks as the relevant factor for the behavior of the chain. We also perform numerical simulations of the structure and properties of these chains; the results of this are in agreement with our analytic predictions. In explaining experimental data for optical properties, such as the second hyperpolarizability, gamma, our theory provides improved agreement over previous models.