The structure of the human body

In this section, I'm going to describe how one can break down the structure of a human body into separate parts, in order to make it easier to remember the position and proportions of its parts. Of course, I neither invented this method, nor do I say it's the best approach possible. But it helped me in the beginning, so you might want to try it out too.
Rather than following everything stated here, you should also read other tutorials and see what works the best for you. I studied several tutorials when learning to draw the body, but sometimes I just ignored the comments, and studied the pictures and photos instead. Now, after looking at a pic, I can already guess the correct sizes, without checking the exact proportions.

Click on the image thumbnails to view larger versions.

Here are the front and side view of the human body. I'll use them to show how to substitute the body by a series of geometrical objects. Also, I'll try to describe the proportions and relationships between body parts.

The proportions

A classic way to remember the vertical positions of body parts is to count "heads". An adult body is usually about 7-8 heads tall. Children are around 6 heads in height. So, you take the size of a head and count how many times it fits into the height of the body.
I'll describe the approximate proportions, counting heads from top. The chin is obviously at the end of head 1. The nipples are around 2 heads from top. At the end of the next head is the navel. The crotch is on half way down, because the legs are half the height of the body. They can also be a little longer though. The knees are about in the half of the legs.

When drawing arms make sure they are not as long as legs, the hands span to the half of the thighs. Some say the elbow should be at the height of the navel (and they are probably right :)), but I draw them a little higher. It looks good so it doesn't matter to me anyway.

If drawing a realistic body, the shoulders (and the rest of the body adequately) should be a little wider than 3 heads next-to each other. If drawing semi-realistic (like me), it's ok for them to be a little narrower (which means the head is wider, depends on how you look at it). The hips are usually narrower than the shoulders, especially for males. When drawing the female body, hips can be almost as wide as shoulders, sometimes even wider.

The structure

Next, we'll look at the at the joints where the body can twist and turn. I marked most of them as crosses, where the lines meet. Besides these, the whole backbone (represented by the vertical line from the head to the abdomen) can be twisted and turned in various ways. The shoulders can be moved a little too. There are restrictions for the joints, obviously. If I'm not sure if the pose I'm drawing is possible (without any bones broken or other injuries), I try it on my own body and see if I can turn into that position.

So at the beginning, the body is represented by a series of lines. I usually replace the head, hands and feet with objects similar to their shapes. Obviously you want a volumetric feeling for your drawings, not just flat shapes. To achieve this effect, you draw using perspective, so that everything gets where it should be. Drawing geometrical objects on places where body parts should be helps a lot. You should learn how to draw simple shapes like a cylinder and cone in space. After drawing the appropriate shapes, you can manage the proportions even in complicated foreshortened views (foreshortening happens when an object is tilted towards the viewer, so that the object seems shorter than it really is). You can see the objects I use as substitutes on the pictures.

So what's the point in making this whole "substitution thingy"? The reason you do this is that it helps you draw the body without having to care for all the minute details. It allows you to sketch the body quickly and fit it into the right composition. After drawing the "fake", you can start drawing in the details and you can be (quite) sure that the proportions and perspective distortions are correct.