Final answer:
A guideline in haircutting is a thin continuous mark, akin to contour lines in drawing, which assists the stylist in maintaining even and precise cuts.
Step-by-step explanation:
A guideline in haircutting is a thin continuous mark used as a guide. This concept is similar to the use of lines in art to create form and guide the viewer's eye. Just as in drawing, where the contour lines define the outline of shapes, such as the outline of a hat on a horseman, guidelines in haircutting help in achieving the desired shape and style by serving as a reference point for cutting hair evenly.
Moreover, lines can be utilized to add depth and dimension through techniques such as hatching, which are closely spaced lines to indicate shading and help an object, like a hat, appear more three-dimensional. Similarly, a hairstylist may use different cutting angles to create layers and texture, adding dimension to the haircut. It is the mastery of using guidelines and understanding how they interact with light and shadow that allows a hairstylist to craft a haircut that looks three-dimensional and well-formed.