A form of painting refers to a monochrome painting that expresses the image of the picture by using lines drawn with pencils, charcoal pens, brushes, etc., or the light and dark tones produced by the combination of lines. Sketching is the basic means for art learners to train their modeling ability, and it is the foundation of all plastic arts. The sketches of many famous painters, like other paintings, are of high artistic value. The subjects of sketches include portraits, landscapes, figures, still lifes, etc. There are full-factor sketches completed over a longer period of time, as well as sketches drawn over a short period of time.

As a product of recording visual images and expressing subjective consciousness, primitive people can record the images of people, animals and plants on rocks and pottery, which can be regarded as the original sketch form. Because it needs to abstract and filter the visual impression of objective things to complete the main expression. The appearance of sketching in the full sense is in the Renaissance after the Middle Ages. During the period from primitive society to the Renaissance, Western painting experienced the pursuit of the real expression of objects in ancient Greece and ancient Rome, and the rigidity and mechanics of the Middle Ages. To express religious consciousness, the form of sketch only exists as a sketch before the work is drawn. Sketch became an independent form of painting from Italy in the 14th century. At first, sketches appeared in the form of studies, copy works, and sketches. By the 15th century, countries north of the Alps had produced a rigorous, detailed style of sketching. At this time, the sketches of northern Italian painters paid more attention to sketches and studies, and their works had the characteristics of freedom and generalization; while the painters of central Italy used relatively certain lines to outline the outline. At this time, sketching began to become a means of apprenticeship in the workshop. During the Italian Renaissance, the expressive means of drawing, such as line, light and shade, perspective, and anatomy, became mature in an all-round way, and the expressive skills were greatly improved. The height of the painting has produced such accomplished sketch masters as Leonardo da Vinci, Michelangelo, and Raphael. The techniques and theories of Italian sketching in the 15th and 16th centuries had a huge impact on other European countries. In Germany, Dürer with a refined and elegant style, delicate and profound, and Holbein, who pursued his style with simplicity and simplicity, appeared in Germany. In the 17th century, the Carracci brothers in Italy founded the Bologna Academy of Fine Arts, which turned the traditional sketch teaching method taught by master and apprentice into a strict academic teaching. During the European Baroque period, there were many outstanding painters known for their sketches.

The French painter Claude Lorrain is famous for his lyrical and idealized landscape sketches. Rubens of Flanders and Rembrandt of Holland are the masters of Baroque sketches, and Tiepolo of Italy. He created the sketching technique of pen and smudge, and Goya from Spain is good at sketching with "red chalk and brush". In the late 18th and early 19th centuries, the importance of line was emphasized in the work of some painters. French painter Ingres' sketches are rigorous and meticulous, full of rational spirit; Delacroix's sketches are free and unrestrained; peasant painter Miller's sketches are as peaceful, simple and kind as mud; Impressionist painters pay attention to the moment. The performance of light and movement, their works appear lively and lively, which stems from the performance of dynamic lines in their works and the generalization of the relationship between light and shade, which is well reflected in Degas's sketches.

Russian sketches in the 19th century made great efforts to create their own sketching system on the basis of learning from Western Europe. The sketching teaching system founded by Cheschakov emphasized the real and in-depth representation of objects through strict and systematic sketching training, and cultivated the Bing, Surikov, Sailov and other sketch masters. In the 20th century, the role of sketch as a creative sketch was weakened, and it developed into an independent creative form. Modernist painters such as Matisse and Picasso all regarded line as the most important part to exert their artistic creativity.

In Chinese painting, line drawing, which purely expresses objects with lines, can also be called a form of sketching. Li Gonglin of the Northern Song Dynasty officially established this form of painting, and many line drawing works of figures, animals, flowers and birds appeared after that. Western sketches were introduced to China in the early 20th century, and were gradually spread and developed by painters who returned to China in the form of running schools. Xu Beihong, Liu Haisu, Yan Wenliang and others who played an important role in the education of modern Chinese sketches, Jiang Zhaohe Incorporating Western sketches into Chinese ink figure paintings enriched the expression forms of figure paintings and had a great impact on the innovation and development of figure paintings.