Unity / variety
Unity is created by the use of elements that look like they belong together, in other words the piece will have a feeling of internal consistency.
The most common ways that Unity is achieved is by using Proximity and repetition. By grouping similar objects closely together they appear to belong with each other. In the image to the right the farm animals and the farm workers are closely grouped together creating a sense of unity, whilst the supervisor on the left of the image is separate both visually and contextually from the rest of the piece.
Repetition of colour, shape or texture can help to create a feeling of unity. A classic example of repetition in art is Andy Warhols Campbell soup mages. These images use repetition of colour, shape and texture to an extreme. Too much repetition in most cases can rapidly become uninteresting visually and so variety is also mportant. The image below (also by Andy Warhol) uses repetition of images to create a sense of unity and variety of colour to create visual interest.