In 1999, the Mountain West Conference split off from the WAC and was given an automatic bid - raising the number of automatic bids to 31. Rather than lose an at-large bid, the NCAA created a the "Play-in game." The Play-in Game matched up two of the lowest-ranked conference champions, with the winner advancing to the NCAA Tournament itself. The play-in games were not considered to be part of the NCAA Tournament; the losers of those games were not considered to have reached March Madness.
In 2001, the "play in" format was replaced with the "Opening Round" game - essentially the same thing, except for the fact that it was considered to be part of the NCAA Tournament. Starting in 2011, the Opening Round was expanded to the "First Four" format, as part of the NCAA Tournament's expansion to 68 teams.