The latissimus dorsi, or “lat” for short, is often referred to as the “swimmer’s muscle”.  It’s the prime mover of arm extension — meaning it does most of the work when you bring your arm back from a forward position.  Such a movement is especially useful in swimming, because by pushing back against the water, it propels the body forward.  To see a well-developed latissimus dorsi, just visit your local swimming pool and look for someone who just swam some “laps” with their “lats”.  You can also use this muscle for pull-ups, or striking a blow with a hammer, but I’d prefer to let mine carry me across a coral reef.