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PolyCAD

When object snapping (OSNAP) is turned on, control points will snap to the geometry of other entities, controlled by the (right-click) popup menu found on OSNAP item on the status. Most of the time it is enough for it to be set to Points and Curves as editing may slow down as the other items are evaluated. Snapping can be turned on with F3 and during editing snapping can be cancelled to allow normal dragging of points by pressing ESC.