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Football 101

HOW MODERN FOOTBALL ACTUALLY WORKS.

A primer for coaches new to scheme, parents trying to follow a game, and anyone who wants the rest of the Spiral AI library to make more sense. Personnel groupings, how the game has shifted over the last 27 seasons (data pulled directly from nflfastR, 1999-2025), and how to read a defense pre-snap.

1. PERSONNEL, THE TWO-DIGIT CODE

Every offensive snap has a personnel grouping. The shorthand is two digits: the first is the number of running backs, the second is the number of tight ends. Whatever's left of five eligible receivers (minus the QB) is wide receivers. So “11” is 1 RB, 1 TE, 3 WR. “21” is 2 RB, 1 TE, 2 WR. “13” is 1 RB, 3 TE, 1 WR.

It matters because the defense matches. Three receivers force a fifth defensive back (nickel). Two tight ends keep base personnel on the field. Coaches use personnel to dictate the matchup before the ball is even snapped.

11
Posse / Eleven
1 RB · 1 TE · 3 WR

The modern NFL's base offense. Three receivers force the defense into nickel.

12
Ace / Twelve
1 RB · 2 TE · 2 WR

Two tight ends, heavier, can shift between run and pass without subbing.

13
Trips TE
1 RB · 3 TE · 1 WR

Heavy package, usually short-yardage or goal-line; forces base personnel.

21
Pro / I-Form
2 RB · 1 TE · 2 WR

The old standard. RB + FB for downhill power runs.

22
Goal-line
2 RB · 2 TE · 1 WR

Eight blockers. Almost always run. Modern usage is rare.

10
Empty (with RB) / Spread
1 RB · 0 TE · 4 WR

Four wide. Forces nickel or dime; primarily pass.

Why coaches care

Defenses match personnel by rule (matching the offense's receiver count) and by tendency. If you see 11 personnel jog onto the field, you can usually predict nickel from the defense. If you see 12, you'll see base. The pre-snap personnel chess match decides which scheme has the matchup advantage before a route is run or a block is thrown.

2. THE ROUTE TREE

Every receiver route descends from one of ten patterns, collectively known as the route tree. They're numbered 0 through 9, with even-numbered routes breaking inside (toward the middle of the field) and odd-numbered routes breaking outside (toward the sideline). Depth generally increases as the number climbs. When you hear a coach call “Z run a 2, X on a 9,” it means slant + go.

This is the common 10-route teaching tree. NFL teams that use Coryell terminology shift some assignments (e.g. 3=Out, 5=Hitch), and Air Raid / Erhardt-Perkins systems use word tags (“Texas,” “Y-Stick,” “Sail”) instead of numbers entirely.

05-6 yd
Hitch

Sprint, stop, face the QB. Quick pitch-and-catch vs soft coverage.

1~5 yd
Quick Out

3-step break to the sideline. Beats off-coverage corners. High-percentage rhythm throw.

25-6 yd
Slant

3-step 45° cut inside. Classic man-beater; against zone, attacks the window between hook defenders.

312-15 yd
Comeback

Push vertical, then snap back down the sideline. Beats off and soft Cover-3 corners.

410-12 yd
Curl

Vertical stem, then sit down facing the QB. Canonical hook/curl-defender beater in Cover 3 and the West Coast curl-flat.

512-15 yd
Out

12-15 yd vertical, then 90° to the sideline. Demands a plus arm; the longest in-air throw in the tree.

612-15 yd
Dig (In)

Square-in across the middle. Sits behind LBs and in front of safeties, eating the Cover-2 hole.

712-15 yd
Corner

Vertical, then 45° to the back-corner pylon. Top Cover-2 beater (high-lows the corner with a flat underneath).

812-18 yd
Post

Vertical, then 45° toward the goalpost. Single-high beater; the skinny variant splits two-deep safeties.

922+ yd
Go / Fly

Straight up the field as fast as you can run it. Takes the top off.

BEYOND THE TREE

Modern offenses are built mostly from concepts that stack two or three tree routes together. A handful of patterns live outside the numbered system and show up on every Sunday tape.

Drag

Shallow crosser at 3-5 yards across the field. Mesh staple; finds zone windows and out-runs trailing defenders in man.

Wheel

Out, then up the sideline. Usually run by a RB or slot vs man, leveraging a linebacker matched on a back.

Seam

Vertical up the seam between zone defenders, run by a TE or slot from inside the numbers. Primary Cover-3 stress; also splits Cover 2.

Bubble

Drop-step, then a curved path back toward the QB. Technically the receiver’s route inside a bubble screen play; gets the ball out fast to a slot in space.

Whip / Pivot

Sell the slant, plant, snap back outside. Whip and Pivot are the same route, used interchangeably. Designed man-beater.

Mesh

Two drags crossing at roughly 5 yards under the LBs. Originated at BYU (Edwards and Chow), popularized by Mumme and Leach in the Air Raid, now a staple at every level.

The names matter, but the job matters more. Slants attack the void between man-coverage hips and zone hooks. Posts split safeties. Curls beat soft zone. When you watch a play break open, ask which coverage rule the route just stressed. That's the whole game.

3. 27 SEASONS, ONE GRAPH AT A TIME

Every number below is pulled directly from nflverse-data play-by-play (the data layer behind nflfastR), covering every regular-season offensive snap from 1999 through 2025. Excludes kneels, spikes, and special teams. 34,429 plays in 2025 alone.

Shotgun adoption
the headline shift since 1999
66.1%
2025, was 12.0% in 1999
19992025

The cleanest single-stat story of the modern game. Shotgun went from a fringe formation to the default in under twenty years, peaking at 72.2% in 2023 before settling. Under-center is now the exception.

Pass-vs-run play call
more stable than the narrative suggests
61.0%
2025 pass, was 61.0% in 1999
19992025

Surprising one. Designed-pass rate (including sacks and scrambles) has lived in a narrow 58-63% band for all 27 seasons. The league isn't passing dramatically more than it did in 1999, it's passing FROM SHOTGUN. The change is structural, not volumetric.

No-huddle tempo
rose, plateaued, came back
9.9%
2025, peaked ~13% in 2014-15
19992025

Hurry-up offense had its moment in the Chip Kelly / read-option / early Patriots era (~2012-2016), climbing past 12% of snaps. Defenses adapted with substitution tactics and the tempo dropped back toward 9-10% before climbing again recently.

What our trend data doesn't cover (yet)

Personnel groupings (11p, 12p, etc.) and nickel-defense usage require NFL Next Gen Stats data that was only added to the public feed starting around 2016. For context: 11 personnel was about 62% of offensive snaps in 2023 (per NGS via Buffalo Rumblings), and nickel was over 65% of defensive snaps by 2022 (per NFL Football Operations). Those numbers can't be extended back to 1999 cleanly.

4. READING A DEFENSE PRE-SNAP

Before the snap, quarterbacks (and coaches in the booth) look at three things: the safety shell, the box count, and the leverage of the corners. Those three pieces of information narrow the possible coverages down to one or two.

Safety shell

1-high = one safety deep, middle of the field closed (MOFC). Usually Cover 1 (man) or Cover 3 (zone). The middle of the field is guarded. 2-high = two safeties deep, middle of the field open (MOFO). Usually Cover 2, Cover 4, or some pattern-match variant. The middle is exposed but the deep halves are bracketed.

Box count

Count the defenders within ~5 yards of the line of scrimmage. 6 in the box , light, run is favored. 7 in the box — even matchup. 8+ — stacked vs the run, pass behind it if you can.

Corner leverage

Press , CB on the line, jam at the snap. Tips man coverage or trap techniques. Off — CB 7-9 yards back. Usually zone, and a hitch or quick out is available. Inside leverage , CB tilted inside, giving up the sideline. Bait for an out or fade.

With those three reads, an experienced QB can narrow eight possible coverages down to one or two before the snap. That's what the cadence is buying time for.

5. GLOSSARY

Terms you'll hear repeatedly across the rest of the library.

MIKE, Middle linebacker, the QB of the defense, identifies the protection points.
WILL, Weak-side linebacker, usually the fastest of the three LBs.
SAM, Strong-side linebacker, the one aligned to the TE side.
3-tech, DT on the outside shoulder of the guard. Primary interior pass rusher.
1-tech / NT, DT shaded on the center. Eats blocks, keeps LBs clean.
EMOL, End man on the line of scrimmage. Sets the edge.
RPO, Run-pass option. QB reads one defender post-snap, gives or throws.
Shell, The safety alignment pre-snap (1-high, 2-high, etc.).
MOFO / MOFC, Middle of field open / closed, based on safety count.
Leverage, A defender's alignment relative to a receiver (inside / outside).
Force, The defender responsible for turning a run back inside.
Cutback, A run lane behind the play-side flow when defenders over-pursue.
Combo block, Two OL temporarily double a DL before one climbs to a LB.
Pull, An OL leaves his spot and runs laterally to lead-block.
Crack, An outside-in block by a slot/outside WR on an inside defender.
Arc, An inside-out lead block, usually by a TE or H-back.
Hot route, A pre-snap receiver adjustment to beat a blitz.
Bracket, Two defenders assigned to a single receiver (inside + outside).

DATA & SOURCES

All trend data on this page is pulled directly from nflverse-data play-by-play parquets (the data layer behind nflfastR), aggregated via DuckDB and committed to the repo as static JSON. Public sources only, no paywalled data. Last refreshed: May 2026 (covering the 2025 NFL season).

  1. [1] Pass rate, rush rate, shotgun rate, no-huddle rate (1999-2025) — nflverse-data PBP releases. Regular-season offensive plays only (kneels, spikes, special teams excluded).
  2. [2] 11 / 12 personnel usage 2023, Next Gen Stats, summarized in Buffalo Rumblings. Personnel data wasn't public in the PBP feed pre-2016, so no 27-year trend.
  3. [3] Nickel usage league-wide (Week 3, 2022) — NFL Football Operations.
  4. [4] Route tree numbering, depths, and coverage stress cross-checked against Matt Bowen's NFL 101, the Ravens playbook tree, and Scouting Academy glossary. Mesh attribution to BYU (Edwards / Chow) per Underdog Dynasty's Air Raid series.

WHERE TO NEXT

With the basics down, the rest of the library will read faster.