Boxing is a combat sport in which two opponents, usually of similar weight, fight using their fists. It is governed by a set of rules and regulations, with the main objective for each of the opponents to knock out their opponent or to win by scoring points when the referee raises the fighters' hands in victory. Boxing is one of the oldest forms of combat sports, having been practiced for centuries in various cultures. It was featured in the ancient Olympic Games, and eventually evolved into a worldwide spectacle that is regulated by organizations like the World Boxing Association (WBA), World Boxing Council (WBC), International Boxing Federation (IBF), and World Boxing Organization (WBO). Boxing matches are usually divided into 12 rounds, each lasting three minutes. Boxers use a variety of punches to try to weaken their opponent, including the jab, hook, uppercut, and other variations. Boxers must wear protective headgear while in the ring. The two fighters are judged by three judges, who decide who the winner is by a count of points or by declaring a knockout. A majority of the judges must agree on the winning fighter in order for the bout to end. The sport gained considerable popularity in the 20th century, largely in part due to legendary and highly-regarded boxers such as Muhammad Ali, Joe Frazier, and George Foreman. Other popular current boxers include Tyson Fury, Oscar de la Hoya, and Anthony Joshua. There are various weight classes in the amateur and professional ranks, ranging from flyweight (up to 112lbs) to heavyweight (unlimited). Some of the professional divisions are subject to the World Boxing Organization, or WBO in a unified set of rules. The boxing year is split into two major seasonal formats, the spring/summer and fall/winter seasons. During each season, boxers must compete a certain number of times and defend their world or regional titles before the end of the season. The season culminates in a Play-off Championship Tournament where the fighters compete for the title of World Champion. Boxing has a long history of attracting large audiences, especially championship matches which may attract millions of viewers. At any time, there are several world champions competing in the professional ranks, each with their own unique style and skills. Amateur boxing also continues to draw interest and has produced many professional boxers, including the likes of Muhammed Ali, Joe Frazier, and Sugar Ray Robinson.