Geometry of a G3L configuration with one source and two lens galaxies; adapted from Schneider & Watts (2005). The two lens galaxies are at angular positions θ₁ and θ₂ on the sky; the source galaxy is at θ. The separation vectors ϑ₁ and ϑ₂ of the lenses from the source have lengths ϑ₁ and ϑ₂, as well as polar angles φ₁ and φ₂. The angle between ϑ₁ and ϑ₂ is the opening angle ϕ. The tangential shear of the source galaxy is measured with respect to the dashed line, which is the bisector of ϕ.