Free overfall weirs are hydraulically efficient and reliable in operation. With this, the overfall capacity depends on the hydraulically active developed length of the weir. Folded types of weir, such as the piano key weir (PKW), provide an over-proportionally developed length and are thus especially efficient, in particular for relatively small energy heads. The compact construction of the PKW in addition allows its economic and rapid creation on crest of gravity dams. Therefore, the PKW lends itself if and when the discharge capacity of existing flood spillways is to be increased or in case a subsidiary overflow is to be emplaced under cramped space conditions. The hydraulic functional capability of the PKW has been tested and optimised by means of several model studios. In addition, several PKW have been built and have confirmed their reliability under flood conditions.