In design, what is the harmonious relationship of parts to each other and to the whole

In design, the principle that can be defined as "the harmonious relationship of parts to each other and to the whole" is UNITY. The design principle of UNITY is present when an artwork feels like on piece, rather than disjointed sections. To truly achieve UNITY these parts work together in a way that creates an overall meaning within the artwork. To create UNITY in an artwork consider using one colour palette throughout an artwork or balancing contrasting colours with connecting features such as a simple motif/design that is present throughout the entire work of art. 

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The answer is all of the above.