Rembrandt Lighting
Rembrandt lighting is a form of lighting that is used for portrait photography. There are two things that make rembrandt lighting, a light on one half of the face, and a triangle of light on the other side of the shadowed face (commonly called the triangle shadow). The triangle shadow is meant to be no longer then your eye and no wider than your nose. it only requires on light source that should be 45 degrees offset from the subject and just a bit above eye level. It is used because is relatively easy to do because it only requires one light source and it can make a photo look natural and interesting at the same time.