Black hole masses from emission‐line widths

Abstract Reverberation mapping and scaling relationships based on reverberation results provide the underpinning of all estimates of quasar black hole masses. When applying these scaling relationships, there are potential pitfalls that are widely unappreciated and can result in biases that can, in turn, lead to systematic errors in the black hole mass function and therefore conclusions about the evolution of black holes over time. Here we discuss specifically the suitability of various line‐width measures for estimating black hole masses.