This article will show you how pistons can be used to make many different automatic doorways. Often they will be 'hidden' to some extent. Pistons can be used to make other passage related elements and these will be included in this article as well. Included in these elements are things such as a pathway over a deep pit, which doesn't exist until a switch is triggered. That passage may even be removed with pistons, after a certain amount of time...
See this world for the examples discussed in this article.