Configuration of the lens system according to the wide (upper panel) and close (lower panel) 3L1S models. In each panel, the cuspy figure is the caustic and the line with an arrow indicates the source trajectory. The three empty magenta circles on the source trajectory represent the source positions corresponding to the three epochs of t₁, t₂, and t₃ marked in Fig. 1. The size of the circle is proportionate to the source size. Lengths are normalized to the Einstein radius corresponding to the total lens mass. The inset in each panel shows a zoomed-out view, in which the locations of the individual lens components (M₁, M₂, and M₃) and the Einstein ring (dotted circle) are marked. The curves encompassing the caustic represent the equi-magnification contours.