Black ink, invented in China, was traditionally used in the Middle Ages for writing, for the simple reason that black was the darkest color and therefore provided the greatest contrast with white paper or parchment, making it the easiest color to read. It became even more important in the 15th century, with the invention of printing. A new kind of ink, printer's ink, was created out of soot, turpentine and walnut oil. The new ink made it possible to spread ideas to a mass audience through printed books, and to popularize art through black and white engravings and prints. Because of its contrast and clarity, black ink on white paper continued to be the standard for printing books, newspapers and documents; and for the same reason black text on a white background is the most common format used on computer screens.