Acupuncture is based around the hundreds of acupuncture points throughout the body. The original textbooks in Chinese medicine only mentioned the points of the body. However, over time, the channels of the body were mapped out and the points were connected. A more complete system was formed as the medicine evolved and progressed. Also, more points were added to cover more areas of the body.
Note that, although there are hundreds of points on the body, points on the arms and legs, below the elbows and knees respectively, are actually the most commonly used points. Most doctors will have smaller group of points which they use commonly and the others are used only in more specific cases.