jats:titleAbstract</jats:title> jats:pThe over tip leakage flow in an unshrouded compressor blade row is highly three dimensional, yet in order for aerodynamicists to analyze and improve designs, they must be able to simplify the problem down to a limited number of mechanisms. In this article, the behaviors of the dominant loss mechanisms are investigated using a multi-order methodology that combines rapid experimental tests of different geometries, detailed measurements in a large rotating rig, large numbers of industry-standard 3D Reynolds-averaged Navier–Stokes (RANS) simulations, and a single direct numerical simulation (DNS) of the datum geometry. The three loss mechanisms identified are ultimately caused by mismatches in flow velocity: separation of the flow as it enters the gap, mixing of the leakage jet with the mainstream close to the suction surface, and endwall shear acting on the jet itself at mid passage. This article is presented in three sections: First, the loss mechanisms are visualized and examined in detail using experiment, simulation, and models. Second, the uncertainty in industry-standard predictions is analyzed and improvements to turbulence modeling are presented. Finally, a matrix of blades with different 3D designs is used to investigate the balance of loss mechanisms and a reduction in total loss generation.</jats:p>