Finish him.

If you were twelve in 1993, you may have known the code for the blood before you knew your locker combination. Mortal Kombat was a crude and gleeful game, and its most famous features were its finishing moves. It also produced one of the more durable pieces of self-government in American business. The ratings printed on every game box began as a way for an industry to avoid being regulated, and they still work that way. Understanding the bargain is the best way to use them.

Mortal Kombat was built at Midway by a team of four: the programmer Ed Boon, the artists John Tobias and John Vogel and the composer Dan Forden. Midway had first wanted a game based on the film Universal Soldier starring Jean-Claude Van Damme, but the actor was not available, so the team went back to its own idea for a fighting game. It later parodied Van Damme with a character named Johnny Cage.

The game reached arcades in 1992. Its fighters were digitized from video of real actors, which made them look like people instead of drawings. That choice, more than the blood itself, is what made the game feel transgressive. Cartoon violence had been in arcades for years. Violence done to figures that looked like photographs of real people was still rare. Each character also had a finishing move called a fatality, triggered by a secret input after a win when the announcer said, “Finish him.” The fatalities were bloody, and they made the game famous.

Acclaim Entertainment released home versions on September 13, 1993, a date its marketing called Mortal Monday, for the Super NES, Sega Genesis, Game Boy and Game Gear. The two console makers made opposite bets. Nintendo’s policies forced the Super NES version to replace the blood with sweat. Sega hid the blood in the Genesis version behind a seven-button code, A, B, A, C, A, B, B, and gave the game an MA-13 rating from its own Videogame Rating Council, which it had announced that May.

The market settled the question quickly. The game sold 3 million copies worldwide within three weeks and topped the American sales charts for 1993, and the Genesis version, blood code and all, was the best-selling console game of the year. The code turned out to be the best marketing Sega could have bought, because every child who learned it told a friend. The forbidden thing was in the box, and a secret password was all it took to reach it.

On December 9, 1993, Senators Joe Lieberman and Herb Kohl opened hearings on violence in video games. Footage from Mortal Kombat was shown alongside Night Trap and Lethal Enforcers. Howard Lincoln of Nintendo of America and Bill White of Sega of America testified for the two biggest console makers. Kohl gave the industry a plain warning: if it did not create a ratings system itself, Congress would do it instead.

The industry chose to govern itself. It formed a trade group, the Interactive Digital Software Association, in April 1994, and the group created the Entertainment Software Rating Board. The ESRB began rating games in September 1994, with categories modeled on movie ratings and short descriptions of the content that earned each rating.

It is worth being clear about what was traded. The industry gave up a measure of freedom by agreeing to label its own products. In return it kept control of the labels, and it kept the government out of the business of deciding what games could contain. The film industry had made the same trade in 1968. It is a familiar pattern: when regulation looks inevitable, the smart move is to write the rules yourself.

The courts later made the arrangement permanent. In 2011 the Supreme Court ruled in Brown v. Entertainment Merchants Association that video games are protected speech, which struck down a California law restricting sales of violent games to minors. The ratings remain voluntary. They are information, not enforcement, and the job of enforcing them at home falls to parents.

The letter on the box is only the headline. Every ESRB rating has three parts: the category, the content descriptors that explain why the game received it, and the interactive elements, which describe online features such as in-game purchases or talking with other players. E, for Everyone, is suitable for all ages. E10+ means ages 10 and up and was added in 2005. T, for Teen, means 13 and up. M, for Mature, means 17 and up and may include intense violence, blood and gore, sexual content and strong language. AO, for Adults Only, means 18 and up.

The interactive elements do not change the rating at all, and this is where the old bargain shows its age. The system was built to answer the question the Senate asked in 1993, which was about blood. The questions parents face now are as often about money and strangers. A game can be rated E for Everyone and still sell upgrades for real money and let a child chat with people nobody in the house has met.

The practical answer follows from the history. The rating summaries on the ESRB website describe the actual content in plain language, and the interactive elements line deserves as much attention as the letter. The parental controls on the console itself can block games above a chosen rating, limit spending and turn off chat. The industry kept control of the labels thirty years ago, and the controls on the machine are the part of that control it handed back to the household.