Adding a second channel to a Fourier analyzer greatly increases its capability by allowing investigation of the relationship between two signals which occur simultaneously. A number of important two‐channel frequency functions can be measured, all derived from the cross‐power spectral function. These include frequency response, ordinary coherence, and sound intensity. This presentation reviews the definitions, physical significance, and some practical applications of these data types. Practical considerations for two‐channel measurements with typical dual‐ or multiple‐channel analyzers are discussed.