A mating structure for a gas turbine engine , the mating structure being configured to rotate about an axis (A-A') of the gas turbine engine , the mating structure comprising: a fastener opening defined by the mating structure and extending through the mating structure, wherein the fastener opening comprises: a circular portion defined by the mating structure and comprising a first radius of curvature (R 1 ); a first slot portion defined by the mating structure and extending from the circular portion circumferentially relative to the axis (A-A'); and a second slot portion defined by the mating structure and extending from the circular portion circumferentially relative to the axis (A-A'); wherein the circular portion is located between the first slot portion and the second slot portion and the first slot portion joins the circular portion opposite the second slot portion ; wherein the first and second slot portions terminate in respective first and second end portions , and the first and second end portions are rounded walls with a second radius (R 3 ) of curvature extending from a fixed point central to each of the first and second end portion wherein the second radius of curvarture (R 3 ) is less than the first radius of curvature; and characterised in that the first and second slot portions extending from the first and second end portions comprise a non-uniform width in a radial direction relative to the axis (A-A') and are curved with a third radius of curvature equal to a distance between a center of each slot portion and the axis (A-A'). a fastener opening defined by the mating structure and extending through the mating structure, wherein the fastener opening comprises: a circular portion defined by the mating structure and comprising a first radius of curvature (R 1 ); a first slot portion defined by the mating structure and extending from the circular portion circumferentially relative to the axis (A-A'); and a second slot portion defined by the mating structure and extending from the circular portion circumferentially relative to the axis (A-A'); a circular portion defined by the mating structure and comprising a first radius of curvature (R 1 ); a first slot portion defined by the mating structure and extending from the circular portion circumferentially relative to the axis (A-A'); and a second slot portion defined by the mating structure and extending from the circular portion circumferentially relative to the axis (A-A'); wherein the circular portion is located between the first slot portion and the second slot portion and the first slot portion joins the circular portion opposite the second slot portion ; wherein the first and second slot portions terminate in respective first and second end portions , and the first and second end portions are rounded walls with a second radius (R 3 ) of curvature extending from a fixed point central to each of the first and second end portion wherein the second radius of curvarture (R 3 ) is less than the first radius of curvature; and characterised in that the first and second slot portions extending from the first and second end portions comprise a non-uniform width in a radial direction relative to the axis (A-A') and are curved with a third radius of curvature equal to a distance between a center of each slot portion and the axis (A-A'). <EOS>