"INTRODUCTION 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"". This Item considers pairs of such angles that define the directions of V and VK with respect to body or normal earth axes. The direction angles of V with respect to body axes are commonly called incidence angles. Relationships are established between these and the attitude angles F@ Q@ Y andFa Qa YaFree-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 in Section 5."