A piano key (PK) weir is a type of nonlinear (labyrinth-type) weir developed specifically for free-surface flow control structures with relatively small spillway footprints. Currently, no generally accepted standard PK weir design procedure is available. This is due, in part, to the large number of geometric parameters and a limited understanding of their effects on discharge efficiency (discharge efficiency is quantified by the discharge coefficient of the standard weir equation). However, Hydrocoop, a non-profit French dam spillways association, has recommended a PK weir design and a head-discharge relationship specific to that geometry. To develop a better understanding of the effects of PK weir geometry on discharge efficiency, 13 laboratory-scale, 4-cycle PK and rectangular labyrinth weir configurations were tested. As a result, the influence of the following PK weir geometries and/or modifications on discharge efficiency were partially isolated: the inlet-to-outlet key width ratio, upstream, and downstream apex overhangs; sloped floors; raising the crest elevation via a parapet wall; fillets underneath the upstream overhangs; and the crest type. The physical model test matrix also included a PK weir configuration consistent with the Hydrocoop-recommended design. From the experimental results, the appropriateness of the Hydrocoop-recommended head-discharge relationship was evaluated, along with the discharge coefficient behavior associated with the standard weir equation. Finally, trapezoidal labyrinth weirs were compared to PK weirs to make a relative comparison of nonlinear weir discharge efficiency; comparisons were made considering crest length and structure footprint.