Etching is a method of engraving where lines or textures are etched into a metal plate, usually copper, using acid. This allows for etching designs onto the plate in a similar way to drawing with a pen or pencil. The earliest etchings date back to the early 16th century, though the technique was used earlier for armor decoration. Notable early pioneers of etching include Albrecht Dürer and Rembrandt, who is considered the greatest etcher. In the 20th century, etching became popular for book illustration.