Piano Key weirs (PKWs) are a hydraulically attractive alternative to linear overflow weirs, increasing the unit discharge at the unregulated spillway inlet for similar heads and spillway widths. This advantage, allowing for operation of dam reservoirs on elevated supply levels thus providing an increased retention volume, is a result of the non-linear nature of PKWs being folded back and forth to make repeating cycles or keys. As PKWs are relatively novel structures, few practical design criteria are available, so that often physical model tests of prototypes are conducted. Nevertheless, first comprehensive and systematic model test series were performed, which provided general design equations. They include the relevant and frequent geometrical dimensions and hydraulic parameters. The paper summarizes recently published design equations relating to the head-discharge ratio of PKWs, and compares the latter with the rating curve of a standard crest profile.