This final entry in the Principles of Design series is all about "Unity", which is the state of balance in your image. Think of your illustration as a sauce -- you don't want any individual ingredient to stand out too strongly. Likewise, no single principle of design should stand out above another, they should all combine into a pleasant mixture. This is called "unity". If you missed any of the previous entries in this series, make sure to check out the Principles of Design Series!