U.S. Open Championship
1895 to 1897 - Stroke play, one day of 36-hole competition
1898 to 1903 - Stroke play, two days of 36-hole competition
1904 to 1913 - Stroke play, two days of 36-hole competition; field cut after 36 holes
1914 to 1916 - Stroke play, two days of 36-hole competition (no championships, 1917-1918)
1919 - Stroke play, two days of 18-hole competition, one day of 36-hole competition; field cut after 36 holes
1920 to 1925 - Stroke play, two days of 36-hole competition
1926 to 1964 - Stroke play, two days of 18-hole competition, one day of 36-hole competition; field cut after 36 holes (no championships, 1942-45)
1965 to Present - Stroke play, four days of 18-hole competition; field cut after 36 holes
U.S. Open Playoff Formats
1895 to 1925 - 18-hole playoff
1925 to 1927 - 18-hole playoff, the day after regulation play
1928 to 1931 - 36-hole playoff, the day after regulation play
1932 to 1953 - 18-hole playoff, the day after regulation play
1954 to 2017 - 18-hole playoff, the day after regulation play; if playoff results in tie, play will immediately continue hole-by-hole until a winner is determined (sudden-death hole-by-hole)
2018 to present - 2-hole aggregate playoff the day of regulation play; if playoff results in tie, play will immediately continue hole-by-hole until a winner is determined (sudden-death hole-by-hole)