These rules are called the Principles of Design.
The Principles are concepts used to organize or arrange the structural elements of design. Again, the way in which these principles are applied affects the expressive content, or the message of the work.
The Principles are:
Balance
Balance is the concept of visual equilibrium, and relates to our physical sense of balance. It is a reconciliation of opposing forces in a composition that results in visual stability. Most successful compositions achieve balance in one of two ways: symmetrically or asymmetrically.
Proportion
Proportion refers to the relative size and scale of the various elements in a design. The issue is the relationship between objects, or parts, of a whole. This means that it is necessary to discuss proportion in terms of the context or standard used to determine proportions.
Rhythm
Rhythm in design is also known as repetition. Rhythm allows your designs to develop an overall consistency that makes it easier for your customers to understand. Once the brain recognizes the pattern in the rhythm, it can relax and better comprehend the rest of the design.
Emphasis
The emphasis in a design is an area or object that is the focal point for the piece. It is a way of making the element that is most important stand out in the design. The focal point may be larger than other elements of the design or brightly colored—both of which tend to draw the eye.
Unity
Unity creates an integrated image in which all the elements are working together to support the design as a whole. A unified design is greater than the sum of its parts; the design is seen as a whole first, before the individual elements are noticed . Unity can be compared to harmony, integrity or wholeness.