Haven't really seen anything discussing the four major portrait lighting patterns in a while so here I go. I am just going to do one at a time, I may be able to get a couple others done today but it is likely I am only going to be able to do this one for now.
First off, the whole idea behind lighting is to create dimension. Flat lighting is just that, flat. It has little to no dimension and does not fair well when you take a 2d image of someone, the only kind you can take with a normal camera. By using light to create dimension you can give the subject a more 3D look.
This is just a standard cube painted gray with a single light 45 degrees camera right and with the light itself pointing 45 degrees down. When I first shot this, I looked at my LCD and said it looked like a 3D render out of my computer.
By knowing about different poses and lighting patterns, you can make someone's face seem less ruddy, more narrow or more broad.
Standard Poses
The four main poses in standard portrait photography are mug, 7/8, 3/4 and profile. Mug is generally considered the least flattering pose while 7/8ths is the most common standard pose.
These diagrams were created by me using a file created by Kevin Kertz. This file can be found here
http://www.kevinkertz.com/fm/LightingSetup.psd.zip
Mug
7/8
3/4
Profile
Lighting
The first standard portrait lighting pattern I wish to show you is
Loop Lighting.
In this little explanation, I will teach you how to correctly meter for the standard ratio what the difference between short and broad lighting is and how to correctly pull off a loop lighting pattern. In this example, the model is 8 feet from a gray background and sitting down in a 7/8ths pose with a 1:3 lighting ratio. A 1:3 lighting ratio is a ratio of 1 2/3 stops of light. That means if the key side of the face is f/11 at 1/60th of a second. The darker side of the face should be f/6.3 at 1/60th of a second. To meter for this, you first meter the key light by putting an incident light meter as close to the face on the key light side. You then point the meter toward the key light and take the meter reading. That is the exposure you use to shoot the image with. You then put the meter on the other side of the face, pointing the meter towards the fill light. Then with your hand you block the light coming from the key light and take a meter reading. If it the key is to dark or to light you can adjust by moving the light closer or further away.
The lighting in this example is a short loop pattern. By short lighting I mean that the side of the face furthest away from the camera is the side of the face lit by the key light. In a broad loop lighting pattern, the side of the face closest to the camera would be the side that is lit by the key light.
Short lighting narrows the face and broad lighting broadens the face.
So in a loop lighting pattern, the key light should be about 45 degrees camera right and above the subject pointing down. The idea is that you should get a shadow of the nose that does not touch the shadow on the left side of the face or the lip of the subject.
Diagram:

As you can see, in this diagram we utilize a key light, fill light, background light and hair light. The hair light should be behind and above the subject and should only fall on the hair. It should not hit the shoulder or face of the person. To achieve this I used a snoot to make the light as directional as possible. The background light should be behind the subject on the ground pointing up.
Result:
This is a better example of the background light, but the pattern is not quite right.
If there are any confusing parts or typos let me know and I will correct them as soon as possible. I typed this up a little to quick for my taste as I have an assignment due later today.