Unity is the underlying principle that summarizes all of the principles and elements of design. It refers to the coherence of the whole, the sense that all of the parts are working together to achieve a common result; a harmony of all the parts.
Unity can be achieved through the effective and consistent use of any of the elements, but pattern-– that is, underlying structure– is the most fundamental element for a strong sense of unity. Consistency of form and color are also powerful tools that can pull a composition together.
However, unity also exists in variety. It is not necessary for all of the elements to be identical in form providing they have a common quality of meaning or style. For example, fashions from a specific period share common features of silhouette, materials, and color that identify the style of the day, or the look of a particular designer.
Unity can also be a matter of concept. The elements and principles can be selected to support the intended function of the designed object; the purpose of the object unifies the design.