SketchUp’s “inference engine” makes it very simple and essentially automatic to connect new geometry (an arc, an edge, etc.) to some existing geometry. Hover the cursor near a location of significance, and SketchUp will jump the cursor to the exact location. Significant locations include end-points and mid-points of edges, centers of arcs and circles, etc. The color of the inference-dot shown by SketchUp indicates the type of location (green for end-points and cyan for mid-points in the current editing context, purple when those locations are in a nested editing context).