What refers to the relationship of facial features and shape to each other?

Facial proportion refers to the relationship of facial features and shape to each other, emphasizing how different elements of the face relate in size and positioning. It is crucial in hair design and aesthetic practices as it helps professionals determine how to best frame and enhance a client's face through hairstyle choices. Understanding facial proportions allows hair designers to create looks that balance and harmonize with the client's unique features, ensuring that haircuts, colors, and styles complement the overall appearance. This concept is fundamental when assessing how to enhance or modify the visual impact of a person's face through hairstyling techniques.

In contrast, facial symmetry focuses on how similar both sides of the face are, which is different from how features are proportioned to each other. Facial design may relate to overall aesthetics and how hairstyles work with the face but doesn't specifically address the measurements and proportions of features. Facial structure deals more with the underlying bone structure of the face rather than the relationships between the features themselves. Each of these concepts plays a role in design but understanding the direct relationship of features specifically is what makes facial proportion the correct answer.