The body weight chair dip, also known as the bench dip or bodyweight dip, is an exercise that primarily trains the triceps muscles. It also involves some pectoralis muscles (chest), anterior deltoids (shoulders), and rhomboids (back). This is an exercise that can be done anywhere as long as you have a platform to place your hands. It is a phenomenal exercise for workouts outside the gym when equipment is limited.
To perform the chair dip place your hips off the edge of the platform so that your outstretched arms are holding up your bodyweight. Your palms should be facing down gripping the edge of the chair or bench. The easiest form of the exercise is with your feet planted on the ground close to your body. Release yourself down under control. Your abs should be tight as you go through a full range of motion. Your shoulders should get to the same level as your elbow, or even below. Push yourself back up and squeeze your triceps as your arms straighten again.
As you move your feet farther from you, the exercise will get progressively harder. You can place your feet on another platform to create more tension on your arms. To increase difficulty after that you will have to add weight to your upper thighs. You may have to have a partner place the weight for you. Make sure it stays on your upper thighs, as close to your hips as possible.