Typically, public interest in having a single, "true" champion resulted in claimants to the heavyweight title being matched with one another; the winner of that bout was subsequently deemed the champion, with the claim (and title lineage) of the defeated boxer largely forgotten.
The prominence of New York City as the epicenter of boxing would lead to a governmental entity, the powerful New York State Athletic Commission (NYSAC), to join the NBA in sanctioning bouts as "world championships."
A third entity, with lesser public recognition inside the USA, the European Boxing Union (EBU), would follow suit, with this triumvirate typically (but not always) recognizing the same boxers as world champions.
At its 1962 convention the NBA's non-U.S. members exploited a membership rule and took control of the organization, rebranding it the World Boxing Association.
A fifth significant (but not as publicly accepted) body came in the form of the International Boxing Organization (IBO), in 1991, and today there are over a dozen sanctioning organizations, of varying degrees of public acceptance, sanctioning bouts as for a world championship and proclaiming their title winners "Champion of the World."