The Piano Key Weir (PKW) is a hydraulically attractive alternative to linear overflow weirs, increasing the unit discharge for similar heads and spillway widths. This advantage allows for operating dam reservoirs on an increased level and thereby provides an enhanced retention volume, resulting of the nonlinear set-up of PKWs being folded back and forth to make repeating cycles or keys. PKWs are relatively novel structures, so that a comprehensive and systematic model test series is presented herein providing design equations. It turned out that primary and secondary parameters exist regarding the relative effect on the discharge rating curve. The main parameters having a significant effect on the capacity are the relative developed crest length and the relative head on the PKW. The secondary parameters of small but not negligible effect comprise the ratio of inlet and outlet key widths, the ratio of inlet and outlet key heights, the relative overhang lengths, and the relative height of the parapet walls.