Millions of people across the U.S. are filling out March Madness brackets this week, taking part in one of the most anticipated annual rituals in American sports.

The odds of filling out a perfect bracket, which means getting all 67 games of the six-round slate plus the First Four play-in games correct, is about one in 120 billion, according to the NCAA, which says it has never been done.

Here is some background on the tournament and how it comes together every year.

What is March Madness?

The single-elimination 2024 NCAA Division I men's basketball tournament consists of 68 teams. 

Thirty-two of the 68 teams in this season's tournament earned automatic bids by winning each of the 32 postseason conference tournaments, which are played in the week before March Madness. Those tournaments were also single-elimination affairs.

The other 36 teams earned at-large bids. As they do every season, on Selection Sunday, which this year took place on March 17, a 10-member Selection Committee made up of various conference administrators and commissioners convened to vote on which schools would receive an invite.

The selection process is based on a variety of factors, such as a team's strength of schedule during the regular season, the quality of its wins — such as whether the wins came on the road or at home, and how the team performed statistically, both on the offensive and defensive end.

It's all part of a complex model known as the NCAA Evaluation Tool, or NET rankings, that the committee has been using since the 2018-19 season.

Going into Selection Sunday, so-called "bracketologists" will list some potential at-large teams as being "on the bubble," meaning that it is a toss-up whether the committee will include them in the 68-team field. 

The major conferences, which include what is known as the Power Five conferences — the Big Ten, the Southeastern Conference, the Atlantic Coast Conference, the Big 12 and the soon-to-be dismantled Pac-12 — tend to get the most at-large bids. This year, for example, the SEC and the Big-12 lead the field with eight schools each in March Madness, followed by the Big Ten and the Mountain West with six schools each. 

What are March Madness brackets?

After whittling down the 68-team field, the Selection Committee seeds them from No. 1 to 68.  

Before the tournament's first round is the First Four, in which the four lowest-seeded conference winners, and the four lowest-seeded at-large teams, compete in what is essentially a single-elimination play-in. Four teams are eliminated, and the four winners of the First Four earn themselves a first-round berth.

The 64-team first-round field is divided into four regional brackets — the South, Midwest, East and West. Within each region, the teams are also seeded from one to 16.  

The No. 1 seeds this year are UConn, North Carolina, Purdue and Houston. 

For context, a 16-seed has only ever beaten a No. 1 seed twice in NCAA history. In 2018, the University of Maryland, Baltimore County shocked the basketball world when it soundly thumped No. 1 seeded Virginia. And last year, No. 16 Fairleigh-Dickinson did it again when it edged top-seeded Purdue. 

Furthermore, only two First Four teams have ever made the Final Four: Virginia Commonwealth University in 2011 and UCLA in 2021, according to CBS Sports.  

How many rounds are in March Madness?

Following the First Four, the NCAA Tournament consists of six rounds: the first and second rounds, the Sweet 16, the Elite Eight, the Final Four and the title game.  

The First Four is being held on March 19-20 in Dayton, Ohio.

The first round kicks off on March 21, and the second round on March 23. The first and second rounds are being played in eight cities: Brooklyn, Charlotte, Indianapolis, Omaha, Pittsburgh, Salt Lake City, Spokane and Memphis.

The Sweet 16 and Elite Eight will be played in Boston, Dallas, Detroit and Los Angeles from March 28-31.

The Final Four and the title game will be held at State Farm Stadium in Glendale, home of the NFL's Arizona Cardinals, on Saturday, April 6 and Monday, April 8.

How long does March Madness last each year?

March Madness lasts about three weeks, starting with the First Four on Tuesday, March 19, and ending with the title game on Monday, April 8.

How long has March Madness been around?

The first men's tournament, which was won by the University of Oregon, was played in 1939 with only eight teams, according to NCAA.com. The tournament expanded to 16 teams in 1951, 32 in 1975 and 64 in 1985. 

A 65th team was added in 2001 to accommodate the Mountain West Conference, and in 2011, another three teams were added to create the First Four. 

According to Statista, North Carolina has the most Final Four appearances in history with 21, followed by UCLA with 18 and Kentucky with 17.  

According to NCAA.com, UCLA has the most titles of any team with 11, followed by Kentucky with eight and North Carolina with six. Of those 11 UCLA titles, 10 came under the late John Wooden, who many consider the greatest college coach of all time. 

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