Users can do different things in different kinds of windows and freely travel though layers of meaning — without losing sight of where they came from and what they are dealing with. An artifact that supports multiple layers of meanings does not merely afford to be looked at from different perspectives, it becomes something different in each semantic layer. The greatest virtue of providing users with different layers of meaning is that they can become experts in the layers they feel comfortable with. They may enjoy one layer without the need to know all others. Users who fear to travel into unknown worlds do not need to learn the languages that define these other worlds. Semantic layers provide choices among kinds of interfaces, kinds of worlds.