Lights at the end of both wings of any airplane
Same as on ships, the navigation lights (green on right side, red on left side, white on aft tip) are there to allow visual judgement about another vehicle‘s position and orientation (and thus relative headings) at night. E.g: 


if you see red and green, you are looking head on at another vehicle
if you see only green, you are looking at the right hand sector of the other vehicle
if you see red and white, you are looking at the left hand aft quarter


and so on.