It was a very early idea that lighthouses could be set up in such a way as to make it more obvious to a mariner which light he was looking at. Rather than merely seeing a light and recognising a danger, the seaman could identify the actual light as a navigational waypoint. Multiple lights could give a very simple form of identification. They could also be lined up to indicate a direction of a safe channel when entering port. This method was called leading lights. Finally, different colours could be used, not only for identification, but also, when shown in specific directions, to help pinpoint the position of a dangerous rock.