The ground line refers to the imaginary line that marks the boundary between the earth and the sky, often used as a reference point in landscape drawings and paintings. It is the starting point from which all other elements in the composition are measured, providing a sense of stability and perspective. In architecture, the ground line is the level at which a building meets the ground, serving as the foundation for the entire structure. In both art and architecture, the ground line is crucial in creating a sense of balance and harmony within a composition.