In the image below, it is clear which elements are related, even though there is no actual content. 

University of Michigan - Ann Arbor

Much like proximity, using similarity in interaction design can help users understand how different elements are related. Those elements can have similar meanings or functions. 

Similarity in Interaction Design

"What are Gestalt Principles?" Interaction Design. https://www.interaction-design.org/literature/topics/gestalt-principles

Gestalt Principles of Design

In the image below, you can see how the email and password boxes are the same size as the login box, however the color of login box demonstrates that it serves a different purpose. Likewise, the blue color of the text at the bottom shows that they are all links.

Learn Before

Related