The 2021 British Open Championship cut line is determined by reducing the 156-player field down significantly heading into the final two rounds at Royal St. George's in England.
The 2021 British Open Championship cut rule is no longer similar to the PGA Tour cut rule, with the top 70 players and ties getting into the final two rounds. The PGA Tour cut rule is now to the top 65 and ties.
There is no 10-shot rule, which means there's no guarantee that a player within 10 shots of the lead qualifies for the weekend rounds. There is a 10-shot rule at the Masters.
There's also no secondary cut if 78 or more professionals make the cut, as was once the case in a standard PGA Tour event.
The cut line will be hovering throughout the second round. From 1968 through 1985, the British Open made two cuts, first to the low 80 and ties after 36 holes, then down to the low 60 and ties after the third round.
Typically, players must be within five shots of the lead heading into the weekend to have a reasonable chance to win.