A nozzle guide vane for a gas turbine engine, the nozzle guide vane comprising: a pressure side wall having a first pressure surface on the exterior of the nozzle guide vane and a second pressure surface on the interior of the nozzle guide vane; a suction side wall having a first suction surface on the exterior of the nozzle guide vane and a second suction surface on the interior of the nozzle guide vane; a radially inner boundary; a radially outer boundary; a leading edge; and a trailing edge; wherein the pressure side wall and suction side wall extend from the radially inner boundary to the radially outer boundary and from the leading edge to the trailing edge; the nozzle guide vane further comprising a single cavity region where the second pressure surface and the second suction surface are spaced apart so as to create a cavity between them, the cavity having a cavity opening point which is nearest to the leading edge of the nozzle guide vane, and a cavity closing point which is nearest to the trailing edge of the nozzle guide vane; wherein the nozzle guide vane has a chord line which is a straight line connecting the leading edge to the trailing edge; wherein, in each plane of constant radial extent between the radially inner boundary and radially outer boundary: a pressure side wall thickness value for any point on the chord line is defined as the distance between the first pressure surface and the second pressure surface measured perpendicular to the chord line at that point on the chord line; and a suction side wall thickness value for any point on the chord line is defined as the distance between the first suction surface and the second suction surface measured perpendicular to the chord line at that point on the chord line; wherein the sum of the pressure side wall thickness value and the suction side wall thickness value for a given point of the chord line between the cavity opening point and the cavity closing point is defined as the combined side wall thickness; and characterised in that the combined side wall thickness varies along the chord line between the cavity opening point and the cavity closing point, such that the maximum value of the combined side wall thickness is at a point on the chord line between 30% and 70% of the chord line. a pressure side wall having a first pressure surface on the exterior of the nozzle guide vane and a second pressure surface on the interior of the nozzle guide vane; a suction side wall having a first suction surface on the exterior of the nozzle guide vane and a second suction surface on the interior of the nozzle guide vane; a radially inner boundary; a radially outer boundary; a leading edge; and a trailing edge; wherein the pressure side wall and suction side wall extend from the radially inner boundary to the radially outer boundary and from the leading edge to the trailing edge; the nozzle guide vane further comprising a single cavity region where the second pressure surface and the second suction surface are spaced apart so as to create a cavity between them, the cavity having a cavity opening point which is nearest to the leading edge of the nozzle guide vane, and a cavity closing point which is nearest to the trailing edge of the nozzle guide vane; wherein the nozzle guide vane has a chord line which is a straight line connecting the leading edge to the trailing edge; wherein, in each plane of constant radial extent between the radially inner boundary and radially outer boundary: a pressure side wall thickness value for any point on the chord line is defined as the distance between the first pressure surface and the second pressure surface measured perpendicular to the chord line at that point on the chord line; and a suction side wall thickness value for any point on the chord line is defined as the distance between the first suction surface and the second suction surface measured perpendicular to the chord line at that point on the chord line; a pressure side wall thickness value for any point on the chord line is defined as the distance between the first pressure surface and the second pressure surface measured perpendicular to the chord line at that point on the chord line; and a suction side wall thickness value for any point on the chord line is defined as the distance between the first suction surface and the second suction surface measured perpendicular to the chord line at that point on the chord line; wherein the sum of the pressure side wall thickness value and the suction side wall thickness value for a given point of the chord line between the cavity opening point and the cavity closing point is defined as the combined side wall thickness; and characterised in that the combined side wall thickness varies along the chord line between the cavity opening point and the cavity closing point, such that the maximum value of the combined side wall thickness is at a point on the chord line between 30% and 70% of the chord line.