The implementation of a new type of labyrinth spillway, called PKW (Piano Key Weir) reveals as a performing alternative for increasing the overflow capacity of existing dams. The optimal hydraulic design of such structure is however not obvious due to the large number of involved parameters. The present work explores experimentally the most relevant geometrical parameters, such as width, length, height and slope of keys and also the upstream and downstream flow conditions. The rating curve of a unit structure is then analysed considering the following dimensionless parameters: (i) the total developed crest length over PKW width (L/W), (ii) the inlet over outlet key widths (Wi/W0), (iii) the vertical over the horizontal shapes (P/Wi), and (iv) the vertical height of the dam over the PKW height (Pd/P). A non-linear global stepwise regression approach is then applied to fit the most influent parameters in order to provide a mathematical formulation for the hydraulic design of a PKW. The discussion reveals that there is not only one optimal solution from the hydraulic point of view. The ideal solution can then only be selected when considering local and economical constraints related to excavation, materials, construction techniques and particularly to the downstream energy dissipation system.