(CNN) In America, the game doesn't start until the national anthem plays. It's not a mandate, it's just a fact, as culturally ingrained as team colors and touchdown cheers.

The anthem kicks off every NFL, MLB, NBA, MLS and NHL game featuring an American team, and is played before every NASCAR race.

The tradition runs deep, and it's easy to forget it wasn't always this way. It's also easy to forget that, while there are game-day expectations about how we should handle ourselves during the anthem, there's very little actually set in stone.

The national anthem wasn't always played before sporting events ...

Here's a fascinating fact: The national anthem was played at baseball games decades before it was actually the national anthem. There are records of "The Star Spangled Banner" gracing the diamond going back as far as 1897, but the song wasn't adopted as the national anthem until 1931.

Over time, a mix of technology, war, and keeping up appearances kept the song in the sports spotlight. Its first big moment reportedly came in 1918 during the 7th-inning stretch of the World Series.

It's no coincidence that its first surge in popularity came during wartime. Nationalism stoked by World War I meant that people were more affected by the song, and the fact that major league baseball players were being actively drafted meant those who weren't drafted benefited from showing their patriotism . Over time, other sports began adopting the practice.

Historian Marc Ferris wrote a history of the national anthem in which he points out that, at first, the anthem didn't exactly come free.

"The thing is, you had to hire a band," he told NPR in 2016 . "That was expensive, so it was only for special occasions," like opening day.

Ferris says that after World War II, sound systems allowed teams and parks to play the anthem sans band. That's when the practice became particularly widespread, because the new tech coincided with a huge swell in patriotism.

Athletes from all sports now line up before games to observe the national anthem.

After World War II, the commissioner of the NFL at the time, Elmer Layden, made a specific plea to keep the anthem as a game-day tradition.

"The playing of the national anthem should be as much a part of every game as the kickoff," he said . "We must not drop it simply because the war is over. We should never forget what it stands for."

... and NFL players definitely didn't always stand on the sidelines for it

While the anthem continued to be a game-day fixture, NFL players typically stayed in the locker room for it.

There were exceptions, of course -- players observed the anthem after 9/11, and during Super Bowl games. But it wasn't until 2009 that players were mandated to be on the field for the song.

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The US Code says you should stand for the anthem

Section 301 of the United States Code says that during a rendition of the national anthem, people "should face the flag and stand at attention with their right hand over the heart" (except for uniformed military personnel, who should stand and salute for the length of the song). If you're wearing a hat you're supposed to take it off.

If no flag is on display, people are supposed to face towards the source of the music and behave in the same manner.

There's a separate US Code section for how to respect the flag, implying that the anthem and flag, while often presented together, are different patriotic entities.

Oddly enough, the US flag code doesn't mention the anthem at all . It does mention the Pledge of Allegiance, which "should be rendered by standing at attention facing the flag with the right hand over the heart."

The field of drivers and their teams stand for the national anthem before the Indianapolis 500 in 2008.

... but there's no such rule in the NFL rule book

In the "2017 Official Playing Rules of the National Football League ," there are no specific directives for what players should do during the national anthem. In fact, neither the anthem nor the flag are ever mentioned.

But what about personal protest? The rule book does touch upon that, but not in any way that could be seen as specific to kneeling during the anthem.

Under Article 8, Section 4, titled "Equipment, Uniforms and Player Appearance," it says:

Throughout the period on game-day that a player is visible to the stadium and television audience (including in pregame warm-ups, in the bench area, and during postgame interviews in the locker room or on the field), players are prohibited from wearing, displaying, or otherwise conveying personal messages either in writing or illustration, unless such message has been approved in advance by the League office.

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In fact, in 2016 the NFL made clear that players were not required to stand for the anthem.

"Players are encouraged but not required to stand during the playing of the national anthem," the NFL spokesman Brian McCarthy said last year after 49ers quarterback Colin Kaepernick's protests gained attention.

For those who wonder if Kaepernick's controversial "pig cop" socks ran afoul of the NFL rule outlined above, they didn't -- he wore those during practices, not on game days.

In August 2016, Colin Kaepernick wore socks depicting police officers as pigs during NFL training camp.

There was money behind some of the NFL's past patriotism

In 2015, a Senate report released by Senators John McCain and Jeff Flake poured cold water on some of the more heartwarming moments of patriotism seen in professional sports.

The report found the Department of Defense had spent $6.8 million on what they called "paid patriotism" between 2012 and 2015. This money was spread out among 50 pro teams from the NFL, NBA, MLB, NASCAR, MLS and others.

In exchange for the money, teams organized displays of national pride including flag presentations, the honoring of military members, reenlistment ceremonies, and even the most unassailable and uplifting of patriotic moments: surprise military homecomings. To be clear, plenty of teams also do, and have done, such things with no compensation.

Some deals between pro sports teams and the Department of Defense included arranging emotional military homecomings (however, not the one pictured).

But the connection between "paid patriotism" and players being mandated to be present for the anthem is tenuous. The report does mention several instances where teams were paid for anthem performances, but that was about the specific artist or presentation. There is nothing in the report to suggest teams were paid or coerced into pulling players on to the field as part of "paid patriotism" initiatives.

In fact, Pentagon spokesman Army Major Dave Eastburn recently clarified the Defense Department's current relationship with professional sports.

"DoD does not require or request that athletes be on the field during the playing of the national anthem when military members are part of the patriotic opener," he said in a statement to CNN.

"Community relations participation, such as flyovers, color guards, and military band support, are unpaid activities. DoD does not pay outside parties to host such community outreach activities."