Gestalt Theory in Typography & Design Principles

http://www.howdesign.com/resources-education/online-design-courses-education/gestalt-theory-typography-design-principles
Shared visual characteristics automatically create relationships. The more alike objects appear, the more likely they are to be seen as a group. Note that similarity is based upon what an object looks like, not what an object is. Two dogs on a page do not automatically have similarity because they are dogs. One could be a Great Dane and the other a Chihuahua—dogs that have very little in common.