It is often necessary to specify the direction of a vector with respect to an axis system. For this purpose only two angles are required, and these are called "direction angles". ESDU 67037 considers pairs of such angles that define the direction of aircraft velocity with respect to body or normal Earth axes. The direction angles of the aircraft velocity relative to air with respect to body axes are commonly called incidence angles. Relationships are established between these and the attitude angles. Free-flight model experiments and wind-tunnel tests are examples of the type of work in which such relations are used. The wind-tunnel application is briefly discussed.