The basics: American football in ten minutes

The goal

Two teams of 11 players share a field 100 yards long and 53⅓ yards wide, with a 10-yard end zone at each end. The team with the ball, the offense, tries to carry it into the opponent’s end zone; the other team, the defense, tries to stop it.

A game has four 15-minute quarters of playing time. Because the clock stops often, a broadcast runs about three hours. The team with more points at the end wins; a tie at the end of regulation goes to overtime.

How teams score

• Touchdown, 6 points: getting the ball into the opponent’s end zone, by running it in or catching a pass there.
• Extra point, 1 point: after a touchdown, a kick through the uprights from the 15-yard line.
• Two-point conversion, 2 points: instead of the extra point, one play from the 2-yard line that ends in the end zone.
• Field goal, 3 points: a kick through the uprights during play, usually when the offense can’t get any closer.
• Safety, 2 points for the defense: tackling the ball carrier in his own end zone.

That is why NFL scores move in threes and sevens, and why margins of 3, 7 or 10 points are so common.

Downs: four tries to gain ten yards

The offense gets four tries, called downs, to gain at least 10 yards. If it does, it earns a first down and a fresh set of four. If it doesn’t, it gives up the ball.

On screen you will read something like “3rd & 4”: third down with four yards to go. On fourth down, teams usually punt the ball away or try a field goal; going for it and failing hands the ball to the other team on the spot.

Every play starts at the line of scrimmage, where the previous play ended. The yellow line drawn on TV shows how far the offense must go for a first down.

Run or pass

Each play begins with the snap: the center hands the ball back, almost always to the quarterback. From there, there are two ways to move forward: run with the ball or throw a forward pass.

Only one forward pass is allowed per play, and it must be thrown from behind the line of scrimmage. If nobody catches it, the pass is incomplete: the ball goes back to where it was and the down is used up.

The play ends when the ball carrier is tackled, goes out of bounds or scores.

Losing the ball

Besides running out of downs, the offense can lose the ball in two ways: an interception (a defender catches a pass) or a fumble (the ball carrier drops the ball and the defense recovers it). These are turnovers, and they decide a lot of games.

Offense, defense and special teams

Each team uses different groups of players on offense, on defense and for kicking plays (special teams). Substitutions between plays are unlimited.

• Quarterback (QB): runs the offense and throws the passes. It is the most important position, which is why his status on the injury report moves predictions so much.
• Running back (RB): carries the ball and helps in pass protection and as a receiver.
• Wide receivers (WR) and tight end (TE): catch passes; the tight end also blocks.
• Offensive line: five players who protect the quarterback and open running lanes.
• Defensive line and linebackers: stop the run and pressure the quarterback.
• Cornerbacks and safeties: defend against the pass.
• Kicker and punter: handle field goals, extra points, kickoffs and punts.

The clock

The game clock stops on incomplete passes, when the ball carrier goes out of bounds (with some exceptions), after every score and on timeouts: each team has three per half. With two minutes left in each half there is an automatic stoppage, the two-minute warning.

Between plays, the offense has 40 seconds to snap the ball.

Penalties

When an official sees a foul, he throws a yellow flag. Penalties are almost always measured in yards, 5, 10 or 15 depending on how serious the foul is, sometimes with an automatic first down. The team that was fouled can decline the penalty if the result of the play suits it better.

The NFL season

The NFL has 32 teams split into two conferences, the AFC and the NFC. Each has four divisions of four teams: East, North, South and West.

The regular season lasts 18 weeks: every team plays 17 games and has one week off. Most games are played on Sunday, plus one on Thursday night and one on Monday night.

Fourteen teams make the playoffs, seven per conference: the four division winners and three wild cards. The playoffs are single elimination and end with the Super Bowl in February.

How to read a game on this site

• Pick: the team the model expects to win, with its win probability.
• Line or spread: the points the market gives the favorite. “Chiefs −3.5” means the Chiefs are expected to win by three or four points.
• Against the spread: the side the model expects to do better than the line suggests. “49ers +3.5” wins if the 49ers win or lose by three points or fewer.
• Total: the combined points the market expects from both teams.
• Keys: the facts that explain the game, written from the stats and the injury report.