The over tip leakage flow in an unshrouded compressor blade row is highly three-dimensional, yet in order for aerodynamicists to analyse and improve designs they must be able to simplify the problem down to a limited number of mechanisms. In this paper the behaviours of the dominant loss mechanisms are investigated using a multi-order methodology which combines: rapid experimental tests of different geometries, detailed measurements in a large rotating rig, large numbers of industry-standard 3D RANS simulations and a single DNS simulation 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. The paper is presented in three sections: First, the loss mechanisms are visualised and examined in detail using experiment, simulation and models. Second, the uncertainty in industry-standard predictions is analysed and improvements to turbulence modelling 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.