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Draw a line perpendicular to the horizon axis, that goes from the top vertex, to a line that is the continuation of the base.
A horizontal line is perpendicular to a vertical line.
It is called the "vanishing point". Your question is about the usefulness of vanishing points when drawing horizontal lines in a painting, and the vertical features of whatever you are painting. It's something you learn in Art lessons.
The perpendicular postulate states that if there is a line, as well as a point that is not on the line, then there is exactly one line through the point that is perpendicular to the given line.
Line L is parallel to line n.