Lines that are perpendicular to the horizon are referred to as which kind of lines?

Lines that are perpendicular to the horizon are referred to as vertical lines. These lines run straight up and down in relation to the ground or the horizon, forming a 90-degree angle with it. In hairstyling, understanding the significance of vertical lines is crucial, as they are often used to create height and give a sense of structure to a haircut or style.

Other options do not fit this definition. Spiral lines are typically associated with curves that wrap around a central point, while diagonal lines create angles and slants rather than standing straight up and down. Curved lines, as the name suggests, follow a bending path and do not maintain a consistent perpendicularity to the horizon. Thus, vertical lines are the only type among the choices that maintain this specific orientation with respect to the horizon.