My write-up below is instructional as a refresher to those that know and new information that are unfamiliar.
In lighting a portrait, ratio and proportion is an important aspect. Begin with the 3/4 lighting or what is known as Rembrandt lighting, named after the Dutch painter who popularized the lighting and build from there. This lighting is also popular as many can make the mistake of just blowing out the subject in an effort to light their subject making the subject look flat and unpleasing.
Wikipedia defines it as:
Rembrandt lighting is a lighting technique that is used in studio portrait photography. It can be achieved using one light and a reflector, or two lights, and is popular because it is capable of producing images which appear both natural and compelling with a minimum of equipment. Rembrandt lighting is characterized by an illuminated triangle under the eye of the subject on the less illuminated side of the face. It is named for the Dutch painter Rembrandt, who often used this type of lighting.
kara_hsIn these particular photos my mainlight is camera left at 1/2 power on an AB800 and a second light the AB400 just behind the model. There is also a reflector at camera right and a smaller round silver reflector underneath the model. Both AlienBees are using medium lightboxes. Exposure info is 1/180th sec at f/9. ISO 200.