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.

Example 1

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

Example 2

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. 

University of Michigan - Ann Arbor

The law of similarity states that "when things appear to be similar to each other, we group them together and we also tend to think they have the same function."

The Law of Similarity

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

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