The wire electro-discharge machining (WEDM) process is widely used in the manufacturing of high-hardness steel precision tooling. Even though the process is characterised by its high accuracy level (sufficient even for micromachining applications), the development of enhanced generators that produce more energetic discharges yielding cutting speeds as high as 500mm²/min has resulted in stronger forces acting on the wire. These forces, together with the low rigidity of the wire, especially in the cutting of parts of high thickness, are responsible for wire deformation that has a direct influence on the accuracy of the part, mainly on wall-flatness and corners. In this work a study on the corner geometry generated by the successive cuts (roughing and finishing) is presented. Errors at different zones of the corner are identified and related to the material removed during each cut. Limitation of cutting speed allows a certain control on the amount of material actually removed by the wire. The influence of different aspects such as work thickness, corner radius and number of trim cuts is discussed. The main conclusion is that a corner accuracy optimisation procedure must consider the errors generated by the previous cuts.