My good old film EOS SII has just a single auto-focus (AF) point and can only focus on vertical or diagonal lines because its AF point had only one bar of photosensitive elements. New SLR cameras have cross type AF points that have two lines of photosensitive elements forming a cross, which allows such cameras to focus on vertical or horizontal lines.
All good things come at a price and cross type AF points are expensive, which is why many entry-level SLR's only have one cross type AF point located at the center of the frame. Remaining AF points are usually sensitive only to either vertical or horizontal lines.
The EOS 7D boasts 19 cross type AF points, which makes it simply great for action photography when you want to track the subject continuously. That is, when tracking a person by keeping the center AF point on their face simply wastes the upper half of the frame. If instead one of the upper AF points is used to track the subject, the frame will be filled more, giving you more image to work with.
This is all nice in theory, but soon after I started using non-center AF points more extensively, I discovered that in many shots the subject is out of focus, while the background doesn't have a hint of blur. At first, I thought I was slipping the AF point off the subject while tracking it, but checking the location of the AF point indicated that it was right where I wanted it to be. Here's an example of such shot. Hover your mouse over the titles to see the location of the active AF point and the close-up crop.
Eventually, I decided to map out crosses inside each AF point, so I can focus on the subject more accurately. I placed a white sheet of paper with a small black square on the wall and moved each AF point perpendicularly to each edge of the black square until the focus light in the viewfinder started to blink, which indicated that I reached the end on the AF cross in the given direction.
Repeating this exercise once for each of the edges gave me four shots where the inner edge of the black square indicated the end of the AF cross in each direction. I superimposed all four shots onto each other and ended up with this picture for the center AF point.
So far so good - the cross inside the center AF point was pretty much what I expected. Note that the cross extends well beyond the red square marker in the viewfinder, which means that if there is more detail outside of the AF point marker, the camera will focus on that detail.
After taking 72 shots and merging each four, I had the complete map of all AF points for my 7D. Let's take a look at the second one form the top in the center column, which is the one used in the sample shot above.
Now it all makes sense - the cross inside this AF point extends well into the top AF point and the camera focused on the detail in the background rather than on the fuzzy police bear.
Here is the complete map of all 19 AF points. Hover your mouse over the numbers to see the AF cross boundaries inside of each AF point.
In order to focus with confidence, you have to memorize the direction of each asymmetric AF point and make sure that the cross inside does not extend beyond the subject, especially if the subject doesn't have much contrast. Note that there is a blind spot between the second and third columns from each side (the ones with the wider gap between them) and the camera will not focus at all if the detail you are trying to focus on happens to be between these columns.