The Inclusion Principle
http://www.alistapart.com/articles/the-inclusion-principle/

Link-Rodrigue, Margit
List Apart, A
2009

Abstract:

Affordance allows us to look at something and intuitively understand how to interact with it. For example, when we see a small button next to a door, we know we should push it with a finger. Convention tells us it will make a sound, notifying the homeowner that someone is at the door. This concept transfers to the virtual environment: when we see a 3D-shaped button on a web page, we understand that we are supposed to “push” it with a mouse-click.