In many cases single-wavelength anomalous diffraction (SAD) phasing leads to a successful structure solution, but it is impossible to predict beforehand if single-wavelength data with a certain amount of anomalous signal resulting will be sufficient. It is therefore safer to continue collecting data at different wavelengths according to a MAD protocol, but to simultaneously attempt to phase the first data set by the SAD method. If this is successful, then further data collection can be abandoned. This `1.5-wavelength’ approach may save a substantial amount of time and effort and diminishes the effects of crystal radiation damage. The principles of SAD phasing are illustrated using vector diagrams in the Argand plane. The IUCr is a scientific union serving the interests of crystallographers and other scientists employing crystallographic methods. Source.