A line is an identifiable path created by a point moving in space. It is one-dimensional and can vary in width, direction, and length. Lines often define the edges of a form. Lines can be horizontal, vertical, or diagonal, straight or curved, thick or thin. They lead your eye around the composition and can communicate information through their character and direction.
Here’s one of my drawings from the Formal Analysis course taught by Peter Eisenman. “Analyze the difference between the corners at Bramante’s Santa Maria della Pace in Rome and Laurana’s Palazzo Ducale in Urbino.”