Alberti’s Veil
A single point of view is required for all forms of drawing what you see. One way of assuring that is by closing one eye. But closing one eye alone will not give you a consistent single point of view. You also need to position yourself in the same place throughout the process of working on the drawing or painting. One of the first to recommend this was Leon Alberti, in reference to what is now known as Alberti’s Veil.