Lens system configuration of KMT-2017-BLG-2509. The inset shows the whole view of the lens system, and the main panel shows the enlarged view around the caustic. The red closed figure is the caustic, the line with an arrow represents the source motion, the two filled blue dots indicate the positions of the lens components, and the dashed circle is the Einstein ring. The gray curves around the caustic represent equi-magnification contours. The empty magenta dots on the source trajectory represent the source positions at t₁ and t₂, marked in Fig. 2. The size of the dot is not scaled to the source size.