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In a league first, the NFL implemented its more flexible flex scheduling rules Thursday, rotating a "Monday Night Football" matchup for competitive reasons.

Moving to ESPN's Week 15 "MNF" stage, the NFC-leading Philadelphia Eagles will now play the Seahawks in Seattle on Dec. 18. That game replaces the Kansas City Chiefs' visit to the New England Patriots, that contest reverting to 1 p.m. ET on Sunday, Dec. 17 on Fox.

The Eagles are currently in the driver's seat for the NFC's No. 1 playoff seed for the second straight year, while Seattle is clinging to a wild-card berth ahead of Thursday night's game against the Dallas Cowboys. The Chiefs are also trying to get home-field advantage and a first-round bye in back-to-back seasons, and playing the lowly Patriots, who are in last place overall in the AFC, will likely propel Kansas City toward that objective.

This season is the first in which the NFL has provided the option to shift Monday night and Thursday night games – on a trial basis, anyway – a mechanism previously reserved for NBC's "Sunday Night Football" when late-season showdowns lost their luster.

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What are the NFL's new flex scheduling rules?

Earlier this year, the league laid out its 2023 flex rules as follows:

▶ For Sunday Night Football, it may be used up to three times between Weeks 5-14, and at the NFL's discretion during Weeks 15-17.

▶ For Monday Night Football it may be used at the NFL's discretion in Weeks 12-17.

▶ For Thursday Night Football it may be used up to twice between Weeks 13-17.

▶ During the Flex Scheduling Windows, the games initially scheduled for Sunday Night Football (on NBC), Monday Night Football (ESPN or ABC), and Thursday Night Football (on Amazon Prime) are tentatively scheduled and subject to change.

▶ Only Sunday afternoon games (or those listed as TBD) are eligible to be moved to Sunday night, Monday night, or Thursday night, in which case the initially scheduled Sunday, Monday, or Thursday night game would be moved to Sunday afternoon

▶ Sunday afternoon games may also be moved between 1:00 p.m. and 4:05 p.m. or 4:25 p.m. ET.

Follow USA TODAY Sports' Nate Davis on X, formerly Twitter @ByNateDavis.