An NFL umpire stands over the ball after an offensive substitution so the defense gets a fair chance to match—while the offense's play clock keeps running.

An offensive lineman reports as eligible to change his number status for one play—but alignment still determines whether he may catch a pass.

The NFL emergency third quarterback dresses but stays inactive until both active QBs are unavailable. Learn the 53-man roster rule and entry limits.

The NFL blue medical tent is a private examination space, not an automatic concussion diagnosis. Learn what happens inside and who decides if a player returns.

The NFL clock stops when a runner goes out, but it may restart before the snap. Learn the two late-game cutoffs and the exceptions viewers miss.

NFL hash marks sit only 18 feet 6 inches apart. Learn why they are narrower than college hashes and how the spacing changes plays, defense and kicks.

Eye black is designed to reduce reflected glare near a player’s eyes. Here is what the research says about grease, stickers, contrast and football performance.

Football players wear towels to dry their hands and protect ball control. Learn who uses them, how they differ from hand warmers and the NFL rules.

The official’s hat is a spot marker, not an automatic penalty flag. Learn what it records and when an NFL out-of-bounds ruling changes the play.

Bo Nix scored on both preseason possessions after two ankle procedures. What Denver can trust now—and what only the next test can establish.