Minecraft wiki entry for Legacy_village_mechanics, associated text: Another way to put it would be to say that an "outside" space has a direct view of the sky, and so has a "SL" (sky light) value of 15. An "inside" space does not have a direct view of the sky (looking straight up) and a "SL" value of less than 15. The "inside" is the side which has more "inside spaces" than the "outside" (which, in turn, has more "outside spaces" than the "inside.") Technically, a sky light as low as 11 (but not lower) is allowed.