Pilot licences (in the Unite States, airman certificates) are issued by national aviation authorities, and establish that the holder has been trained by a qualified flight instructor and has met a specific set of knowledge and experience requirements, sometimes determined by taking a checkride. The licensed or certificated pilot can then exercise a specific set of privileges in the nation’s airspace. Despite attempts to harmonize the requirements between nations, the differences in certification practices and standards from place to place serve to limit full international validity of the national qualifications.