Lens system configurations of OGLE-2017-BLG-1691: upper panel (inner solution) and lower panel (outer solution). The inset in each panel shows the whole view of the lens system, and the main panel presents the zoom-in view of the caustics. The red cuspy figure is the caustic and the line with an arrow is the source trajectory. The orange circle on the trajectory is drawn to represent the source scale with respect to the caustic. The blue solid dots marked by M₁ and M₂ indicate the positions of the host and planet, respectively. The dashed circle with unit radius in the inset represents the Einstein ring.