A theory is developed which predicts the interference velocities and interference velocity gradients caused by the walls of the tunnel. Large wake deflections are allowed in both the lateral and vertical directions. The theory includes V/STOL and conventional wall-interference theories and ground effect as special cases. Symmetry and interchange relationships between the interference factors are developed and extensive numerical results are presented. Use of the interference factors to correct data depends upon the availability of detailed aerodynamic treatments in nonuniform flow of the model under test. In most tests the available aerodynamic treatments will be found either inadequate or too time consuming for rigorous routine correction of data relating to lateral-directional stability.