It is a line parallel to the picture plane base and the ground:

This line is known as the "horizon line" in perspective drawing. Here are the steps to understand it:

1. The picture plane is an imaginary flat surface that is assumed to be identical to the surface of the painting. It is perpendicular to the line of sight.

2. The ground plane is the horizontal plane that extends outward from the picture plane. It is where the objects are located.

3. A line parallel to both the picture plane base and the ground would be the horizon line. This line represents the viewer's eye level and is where the sky meets the ground.

4. In perspective drawing, all parallel lines converge to a point on the horizon line. This point is called the vanishing point.

5. Therefore, the horizon line is a fundamental element in creating the illusion of depth and distance in a drawing.

This line is known as the "horizon line" in perspective drawing. Here are the steps to understand why:

1. In perspective drawing, we often start by establishing the horizon line. This line represents the viewer's eye level and is where the sky appears to meet the ground.

2. The horizon line is always at the viewer's eye level - its placement depends on the perspective of the viewer. If the viewer is looking straight ahead, the horizon line is usually in the middle of the scene. If the viewer is looking up or down, the horizon line may be lower or higher in the scene.

3. The horizon line is parallel to the ground because it represents the viewer's eye level. No matter how high or low you are, your eye level is always parallel to the ground.

4. The horizon line is also parallel to the base of the picture plane because the picture plane is a vertical plane (imagine a glass window) through which the viewer sees the scene. The base of this plane is at the same level as the viewer's eyes, so it is parallel to the horizon line.

5. Therefore, a line that is parallel to both the picture plane base and the ground is the horizon line.