Two types of lines exist in art: static and varied. An artist uses both kinds of lines to give the viewer visual clues about the nature of the object on the canvas. Static lines in a drawing have equal and consistent visual weight; they all tend to be about the same width. The use of this line tells the viewer that the subject she's looking at is solid, almost unmovable. Varied lines are the opposite of static lines; they possess various thickness to show different visual weights. When used correctly, these lines can indicate the visual weight of an object. Both types of lines are used in well-composed pictures.
According to the Goshen College website, composition in art arises out of all of the elements of a piece working together to create a balanced and harmonious whole. Line plays an important part in this, because it represents one of the basic building blocks of picture making. Line quality plays a significant role in the overall compositional structure of a piece. For example, a heavy line drawn at the bottom of a tea cup gives the cup visual weight --- so much so that if the cup were only drawn with a series of lines, the visual weight of the cup would look correct even if there were no shading to provide a more realistic three-dimensional look. As well, if the line defining the rim of the same tea cup were drawn so that it was very thin with spots where the line almost breaks apart, the viewer would get the sense that a highlight was there and that the material of the cup was thinner in that area of the cup. These two types of lines working together make the cup look balanced on the page.
The direction of a line --- whether it's drawn at a diagonal as opposed to vertical or horizontal --- will visually indicate movement as well as being a tool that an artist uses to create an almost three-dimensional quality to his drawing or painting. According to the J. Paul Getty Museum, the diagonal line has this effect because of the psychological components of lines. A horizontal line produces a calming effect, because it replicates the horizon or the line of the ocean, whereas the vertical conveys weight and permanence as in the case of a tree in a drawing. Because diagonal lines go against these visual rules, they create a more dynamic composition.
Line is one of the primary tools an artist uses to create not only gesture in drawings or paintings, but also textures. Artists use a technique called gesture drawing, which is a series of scribbles on a page. These help the artist fill in the details of a drawing. The artist also uses lines to create textures as varied as leather is from silk. Where these lines are placed on the picture plane --- at the top, bottom or middle --- moves the eye along the composition and informs the viewer of how heavy an object is. For example, the texture of a leather bag is largely composed of a series of short, scribbled strokes with a graphite or charcoal pencil. These lines take the gesture drawing one step further, but are based on a similar principle. The lines fill in the details of how heavy the leather bag is, but also give a clue as to where the bag is on the picture plane. A bag placed in the foreground of the drawing would most likely have lines that looked closer together, whereas the bag in the middle or top of the picture plane would display more tightly woven lines, making it look farther away.