In hair design, lines that create the illusion of length are referred to as:

Lines that create the illusion of length in hair design are known as vertical lines. These lines run straight up and down, influencing the visual perception of length and providing a sense of height to the overall hairstyle. When vertical lines are incorporated into a haircut or style, they help elongate the look of the hair, which can be particularly flattering for various face shapes and overall appearances.

On the other hand, horizontal lines tend to create a sense of fullness and width, making the hair appear broader rather than longer. Diagonal lines can introduce movement and interest but do not specifically promote the effect of length. Curved lines add softness and encourage natural movement but also do not primarily focus on enhancing the vertical aspect of a hairstyle. Thus, vertical lines are specifically the ones that contribute to an elongated visual effect in hair design.