Final answer:
The principle that holds all elements of hair design together is harmony, which creates a visually pleasing and cohesive look.
Step-by-step explanation:
The principle of hair design that holds all the elements of design together is harmony. Harmony in art and design refers to the visually satisfying effect of combining similar, related elements. For example, adjacent colors on the color wheel, similar shapes, or a consistent style of brushwork can all contribute to the harmony of a piece. In hair design, harmony represents the cohesive look that a hairstylist aims to achieve when various elements such as color, shape, and texture work together in a pleasing manner, thus creating an aesthetically cohesive look.