Boundary lines are the points at which the screen ends in a level of World of Goo. Boundary lines can be harmful, helpful, and/or harmless. There are four different boundary lines, one for each of four directions: right, left, bottom and top. Boundary lines only affect Goos if nothing is blocking them; for instance, if there are walls on the side of the level, Goos cannot touch the side boundary lines. If there is a level with water, the Goos will not be affected by the bottom boundary line. Boundary lines can either destroy Goos, break them apart or do nothing. Boundary lines, like blades, pose a large threat to players trying to achieve OCD, as once a Goo is destroyed by a boundary line, unless using a time bug, there is no bringing it back.
Side and top lines
Some side and top boundary lines destroy Goos.
These sometimes break apart non-detachable Goo structures, such as in Tower of Goo.
Goo Balls will not go past these lines if they are harmless. They will break apart structures and then go back.
If reachable, and not water, will kill Goos indefinitely, except in the secret part of Beauty and the Electric Tentacle.