From charcoal, pencils, pens, etc., lines are used to draw a picture of a dark and monochromatic painting called a sketch. Sketch is the basis of painting.
Sketch has the law of natural change. Different pens create different lines and cross-cutting relationships and rhythms, active and passive surroundings, planes, volumes, tones, and textures.
Sketch is a formal artistic creation that expresses things in the intuitive world with monochromatic lines. It can also express ideas, concepts, attitudes, feelings, fantasies, symbols, and even abstract forms. It does not focus on the overall and color as the color of the painting, but on the structure and form.