Listing Requirements
  
Every club has its rules. You have to be 30 years old to get elected to the U.S. Senate. You have to be a superhero to join the Justice League. And so on.
Along the same lines, listing your stock on a public exchange has certain prerequisites. These rules represent the exchange’s listing requirements. They differ from exchange to exchange (the NYSE might have different rules than the London Stock Exchange, etc.). A company needs to meet the listing requirements in order to sell its shares on a particular exchange. If the stock falls below some requirement, the company will usually get some sort of warning. But if the problem isn't fixed, the stock could face delisting.