The US Open playoff format has been changed for 2018...
What Is The US Open Playoff Format?
What is the US Open playoff format if, after the regulation 72 holes, we have a tie at the top of the leaderboard?
The answer used to be an 18 hole playoff. Assuming the tournament finishes on the Sunday, the players tied at the top would then come back the next day to play 18 more holes.
This last happened in 2008 when Tiger Woods and Rocco Mediate played 18 extra holes on Monday at Torrey Pines but they tied at level par so had to go into sudden death.
Woods went on to win on the first extra hole.
This year, however, the US Open playoff format has undergone huge changes.
No longer is it an 18 hole playoff, to the delight of fans and broadcasters who all want the tournament to finish on the Sunday.
Starting from this year, the US Open playoff format will be two hole aggregate.
This is still unique to the other majors which all have different playoff formats of their own.