A fundamental relationship of inherent optical properties (IOP) is that the beam attenuation coefficient is the sum of the volume absorption and scattering coefficients (c=a+b). A relative calibration of a set of instruments can be provided using this IOP closure equation. Measurement of the true beam attenuation coefficient c is not practical as all attenuation instrumentation has some finite acceptance angle in which scattered light is collected. We provide a theoretical framework for measuring the attenuation and scattering coefficients in a consistent manner. Using this framework, we provide a practical version of the IOP closure equation. We apply the practical IOP closure equation to measurements made at Lake Pend Oreille, Idaho, in the spring of 1992. Results of this IOP closure indicate that the practical closure equation is a useful approach. Closure was achieved during some measurements sets but not at others. The intermittent lack of closure may be due to the method of determining the scattering coefficient from the general angle scattering meter or that the calibration of at least one of the instruments drifted during the time of the experiment. ¿ American Geophysical Union 1995