Chin-up bars are playground equipment that were once ubiquitous on children's playgrounds. They are still important in the adult equivalent of a playground, the Par course. A chin-up bar is simply a smooth horizontal metal bar, often a pipe, held solidly above ground by a wooden or metal frame. Typical installations include 2 or 3 different heights of bars for people of different heights.
In its common usage, a person jumps up slightly to grab the bar in both hands so that the palms are facing away and the feet hang freely in the air. The exerciser then pulls himself up to where his chin passes the top of the bar, slowly lowers himself to hanging by his arms, and repeats as many times as possible. This is referred to commonly as a pull-up.
The chin-up can also be performed using an inverse grip, where the palms of the hands are facing the participant. This is what is commonly referred to as a chin-up. This type of grip usually places more emphasis on the intercostals and the biceps, whereas the traditional grip is more of an upper-back and latissimus dorsi exercise.
Further variations on chin-ups are possible by gripping with only a few fingers of one hand in order to increase resistance on the other arm. This type of exercise should be balanced evenly on both arms. One-armed chin-ups are also possible but are notoriously difficult to achieve.
Chin-up bar counts were once a part of the U.S. President's Council on Physical Fitness program for evaluating the physical health of schoolchildren in the 1960s and 1970s.
Children found other creative ways to use them, however, such as hanging by the knees, pulling oneself up to the top and sitting on them (more common with monkey bars variation), and so on.