When it comes to art, "Unity" maybe the closest thing we have when it comes to a rule for art. In design, Unity means an agreement of elements in a design; they look as they belong together and is something beyond chance that has brought them together. Another word for this is harmony. So ensuring this concept in design may seem like a no brainer, but it is still something that has to be remembered when designing.
A great example is a study by Artist Wayne Thiebaud titled "Four Pinball Machines - Study". This painting shows a great example of unity. Besides the fact that all of the elements are similar, the similar shapes and the negative spaces between elements also demonstrate unity.
The different ways we can achieve unity in design are as follows:
Wayne Thiebaud is a Northern California artist that made it big during the sixties during the Pop Art movement. He is considered an American Master and is famous for his cake paintings. He of course is more than the cake paintings that made him big. He is also a teacher and a producer of other great work. For more info on Wayne Thiebaud, please see the following Smithsonian article and videos below.
*Content taken from "Design Basics" (Rinehart and Winston Inc.) Publishers.
Artsy Wayne Thiebaud Page
For more information on Wayne Thiebaud, check out the following Artsy page with images of more of his work and other articles about him.