In a two-dimensional electron system the orbital effects of in-plane magnetic fields are commonly neglected. This is to ensure that the Hamiltonian is separable and the system can be described in two dimensions. However, for wide quantum wells and/or large in-plane magentic fields this approximation lacks accuracy and the third dimension has to be re-established. The present work discusses corrections to the two-dimensional approximation and generalizes the one-particle description of the integer quantum Hall effect. The main focus is on coincidence measurements and possible transport anisotropies due to spin-orbit coupling.