there's a special rule dictating that teams can have 45 players suit up for every game, plus a 46th player, known as the emergency quarterback, who can only play if the first- and second-string quarterbacks have gotten hurt.
The rule has confused some people because of the "Entry in First Three Periods" clause. The provision states that if a third quarterback is inserted before the fourth quarter, a team's first two quarterbacks cannot be used for the rest of the game at any position.
"The purpose was to give teams an emergency quarterback," Signora said. "That's the point of the basic rule. Provisions to the rule were put in - like the fourth-quarter one - to keep teams from essentially expanding their rosters to 46."
If there were no clause, a coach, in theory, could list any player as the third quarterback, bring him in at any time, and let the starting quarterback resume his role.
While the emergency quarterback isn't technically active, he may dress and participate in all pregame and halftime warm-ups. If he enters the game, he must line up under center or in the shotgun.
At that point, "he may run, throw a pass, quick-kick, or catch a pass, if he becomes an eligible receiver," the NFL handbook states. After the first play, he must continue to play quarterback. He also may be used as a kicker, punter, or holder in kicking situations.