An old English property law, originating in 1663.
It was drafted to protect the rights of homeowners and tenants to safeguard the amount of daylight that they usually enjoyed. If a window was used for lighting for a period of twenty years, it was forbidden for any neighbouring landowner to construct any building that blocked the light.
Cool name for a law, huh? I bet you were expecting a poem, or something.
You can see a picture of such a sign round the back of a top britnoder-couple's flat- http://www.wertperch.co.uk/gallery/view_photo.php?set_albumName=hell_hath_no_furry&id=Ancient_lights