Six gravitational resonances amongst planets and satellites are thought to exist in the Solar System, and four such resonances have been found in other planetary systems. However, no fully general, exact analytical model describing gravitational resonances has been developed. Hamiltonian constructs and the pendulum paradigm have served as benchmarks for describing the evolution in and reach of orbit-orbit resonances. I have created a model describing the latter, and obtained analytical expressions for libration widths that include a variety of effects, including central-body oblateness, a surrounding thin disk with gaps, short-period contributions from non-resonant terms, and general relativity. I will derive this model and apply it to all known resonances amongst large bodies in planetary systems.