In this detail of a print by Albrecht Dürer, notice how contour lines are used to mark the outside of the man’s face and hat, and to define his facial features.
Note, though, that lines are also used to show shading — the shadows caused when light comes from one side, leaving the other in shadow. On his hat, for example, the closely spaced lines, called hatching, show that the left side of his hat is in a shadow. This also helps the hat to look more three-dimensional.
This detail is from another Dürer print, now showing the Four Horsemen of the Apocalypse. Now, you can immediately see the contour lines outlining the figures, and also the hatching lines creating the illusion of shading and depth. But what are those horizontal lines in the background doing? They do create shading, but they also help create the sense that the riders are moving rapidly from left to right. Motion lines are familiar from comic strips, but they appear in all sorts of work.
We can also look for implied lines, which are not actually drawn, but we can connect the dots (literally or figuratively) to create the lines in our minds. Leonardo da Vinci’s Virgin of the Rocks contains wonderful examples.
Here, the implied lines are sight lines. We can follow the gazes of the figures as they look and point at one another.
The angel in the red cape to the right looks out at us, and then points at the infant John the Baptist, at the left. He looks at the infant Jesus, who in turn looks back again at him. Above, Mary looks down at Jesus, and also gestures toward him with her hand. Basically, once we make it into the space of the painting by meeting the gaze of the angel, we become locked in a cycle of movement between the holy figures, guided by their sight lines.