The very basis of the theory of thin wings is the (integral) relation between the velocity field in the irrotational exterior of the wing and its wake, and the intensity of the sources that generate it. These sources occupy the fictitious interior of the wing-wake entity, and their intensity can ultimately be related with the pressure jump across the wing. When combined with impermeability condition on the surface of the wing, the integral relation between the velocity and the intensity of the sources yields an integral equation for the intensity of the sources. This chapter presents a formal derivation of its linearized form.