A method of forming a turbine nozzle guide vane 2 for a gas turbine engine, the guide vane 2 comprising an internal cavity 14 and having internal features 16, comprises investment casting metal around a core member 30 that defines the internal cavity 14 and internal features 16 of the guide vane, removing the core member 30, and machining the guide vane 2. The process of investment casting around the core member 30 includes the step of casting at least one feature 40 external to the guide vane 2. The external feature 40 is used as a reference point to indicate the location of the internal features 16 of the guide vane 2 during the machining of the guide vane 2.