To fill in that dark area opposite the key, a second light called the "fill" is used. The fill light is placed like the key,  except that it is placed on the other side. So if the key is on the subject's right, the fill should be on the left.

The trick here is to ensure that the fill is NOT as bright as the key. If both were the same brightness, the result would be "flat" lighting. Flat lighting means the light is even everywhere.

In the image at the right, note how one side of the actor's face is brighter than the other. The difference is not stark like in the previous page—it's more subtle. But there is a difference.