A diffractometer‐independent definition of the azimuthal angle ψ of a Bragg reflection is proposed. After completion of intensity measurement, instrument‐dependent and partly redundant quantities such as orientation matrix and setting angles may be replaced by ψ, values without loss of information. Data‐treatment software, such as absorption‐ and anisotropic extinction‐correction programs, become instrument independent and completely general. Formulae are given to compute ψ for equatorial and non‐equatorial diffraction geometries.