What is March Madness? The popular US basketball tournament explained

Across the US every March, the flowers bloom, the trees grow new leaves — and everyone is glued to their TV screens for March Madness, the annual college basketball tournament.

The tournament is run by the National Collegiate Athletic Association (NCAA) and pits the nation's top university basketball teams against each other in a fierce competition that has taken place almost every year since 1939.

The competition was cancelled last year to the Covid-19 pandemic, the only time in history it was not held.

It is a single-elimination tournament of 68 teams that compete in seven rounds for the national championship, with the second-to-last round known as the Final Four, when only four teams are left.

There are two ways that a team can earn a bid to the tournament, according to the NCAA. The 32 Division I conferences all receive an automatic bid, which they each award to the team that wins the postseason conference tournament.

Auburn's TJ Dunans shoots between Tennessee's Robert Hubbs III and Detrick Mostella during the first half of an NCAA college basketball game on March 9, 2016. AP

The second way is receiving an at-large bid, which goes to the highest-scoring eligible teams across different levels, meaning even schools without big, expensive athletic departments have a chance to play.

Once the field of 68 teams is finalised, they are assigned a seed and sorted into four regions, which determines their first-round matchups and their path to the championship, the NCAA says.

The tournament is wildly popular and even those not normally interested in college sports participate.

The big draw for fans isn't necessarily free throw strategies or hook shots, however — it's the brackets.

At the beginning of every tournament, fans