What essential feature does a perspective sketch require?

A perspective sketch is fundamentally characterized by the concept of a vanishing point. This intrinsic feature allows for the representation of depth and three-dimensionality on a two-dimensional surface. The vanishing point is where parallel lines appear to converge as they recede into the distance, mimicking how the human eye perceives the spatial arrangement of objects. This technique provides a realistic representation of the spatial relationships and proportions of elements within the design, making the sketch easier to understand from a viewer's perspective.

In contrast, while a scale can be useful for accuracy in measurements and creating proportionality, it is not an essential feature of perspective drawing itself. Similarly, a legend may help clarify symbols and components within a drawing, but it does not contribute to the fundamental perspective aspect. Sectional details provide insights into the interior composition and structure, but they are not directly tied to the perspective representation that conveys depth and spatial relationships. Therefore, the vanishing point is critical for achieving the visual effects inherent in perspective sketches.