US Open sectional qualifying is one of the best days in golf. For a few years, the USGA marketed the international day of sectional qualifying as Golf's Longest Day, and it truly is. The US Open sectional qualifying format is brutal for every player, regardless of whether they're a former major winner, a touring professional, a club pro or a high-level amateur.
The US Open sectional qualifier format is a 36-hole stroke-play tournament. The combined scores from the morning and afternoon rounds represent a player's score, and the players posting the lowest scores inside the number of US Open qualifying spots allotted to a specific site by the USGA then earn a spot in the US Open. For example, if the USGA allots 10 US Open spots to Sectional Qualifier A, the top 10 players who post 36-hole scores get into the US Open. It's that simple.
However, the USGA doesn't typically tell players until the day of how many US Open spots they're playing for in their respective sectional qualifiers. This means players find out just before getting going how well they have to play in order to get in the national championship.
In the event at the end of the 36-hole tournament there are more players tied for places atop the leaderboard than there are allocated US Open spots, then there are immediate playoffs to determine the spots and the first and second alternates from each site. For example, if there are 10 spots available and four players are tied for ninth place, all four players tied for ninth place go into a sudden-death playoff to determine which two players get in the US Open and which are alternates. In these playoffs, players can both win and be eliminated. Back to the example, if, on the first playoff hole, one player makes birdie, two make par and one makes bogey, the player making birdie earns the first spot available, and the player making bogey is eliminated and becomes second alternate. The remaining two players who made par now continue in their sudden-death playoff to decide the second spot and first alternate.
Ultimately, alternates matter as the USGA typically uses a handful of alternates to fill in the 156-player field when all allotted spots for exempted players are not filled. These alternates are ranked mostly based on the site where they finished alternate, with those earning alternate spots from sectional qualifiers with the biggest and most talented fields getting preference.