At the 36-hole mark in most PGA Tour events, there's a cut down to the top 70 and ties.
Sometimes, the top 70 and ties constitutes too many players, and the Tour doesn't want to create a logistical nightmare when more than half the field makes the cut. So, since 2008, the Tour has implemented a secondary cut after the third round (through 54 holes) to limit the field for the final round.
When at least 78 professionals (amateurs don't count) make the second-round cut at a PGA Tour event, the secondary cut kicks in, reducing the field after three rounds to the number nearest 70 as possible for the last round. That happened on Saturday at the Sony Open in Hawaii. After 87 players made the cut on Friday at Waialae Country Club, the field was reduced again after the third round to 74 players, leaving 13 players without a Sunday tee time.
However, all is not lost for those players who got the MDF (made cut, didn't finish) treatment. They all get paid like they made the cut and get FedEx Cup points. They just don't get a chance to improve on their standing in the final round.
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During the FedEx Cup playoffs, however, the secondary cut rule does not apply when in The Barclays and Deutsche Bank Championship, the two events in the series that do have a 36-hole cut.
The European Tour adopted their own secondary cut rule in 2017. The European Tour secondary cut rule is also for when 78 or more pros make the cut, making a third-round cut to the top 72 players and ties.