The official rules of polo strictly prohibit players from hitting the ball using a mallet held in their left-hand. The rule is designed to protect players from clobbering each other during a polo match.
The "no left-handed player" rule was originally implemented during the 1930s, but relaxed around the time following the end of World War II. However, the United States Polo Association reinstated the rule for good in 1974.
Polo takes place on horseback and is a team sport. Similar to hockey and soccer, the idea in polo is to score goals, which are located on either end of a polo field. The ball is moved through the use of a long-handled wooden mallet.