Welcome to Football 101 presented by The Painted Lines! In this ongoing offseason series, I will be breaking down football concepts to help you better understand what you are watching on Sundays. Each article will take a deep dive into a specific component of the game.  We will also be updating a cheat sheet throughout the series that you can bookmark or print out for quick reference on game days. You can also join our public discord to interact with our writers and podcasters and talk X’s and O’s.

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Without further ado, let’s dive in!  Today we are going to take a look at defensive coverage shells.


There are 2 basic types of coverages in the NFL: man-to-man defense and zone defense. In man-to-man defense, each defensive player is tasked with covering one offensive player. In zone defense, each defensive player is responsible for covering a certain area of the field, and thus any play that enters that zone. Generally speaking, there are 3 types of man-to-man coverage (cover 0, cover 1, and cover 2) and 3 types of zone coverage (cover 2, cover 3, and cover 4). If that’s confusing don’t worry about it. In later posts, we will talk more specifically about these different coverages, but for now, you just need to know those 2 basics.

When an offense lines up at the line of scrimmage, the defense lines up in their called defense and the QB will look for any clues about what defense the opposing coach has called. The first thing that he will look at is the safety alignment or coverage shell (i.e. how the safeties are aligned on the field). Why the safeties? There is an old coaching adage that says, “cornerbacks lie, but safeties tell the truth.” It is much harder for defenses to disguise what their safeties are doing, and a good quarterback can take the information that the safeties give away and use it to figure out the rest of the defense underneath. 

There are 3 coverage shells that the defense can be in, and each gives the QB clues about what defense has been called.

MOFC (Middle of Field Closed) or Single High Safety

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This is called a MOFC defensive shell or a single high safety look. It gets its name because there is a single safety away from the line of scrimmage who closes off passing concepts attacking the middle of the field. See? Football isn’t that hard! In this shell, the free safety is away from the line of scrimmage (circled in the photo) while the strong safety is up near the line of scrimmage (the arrow in the photo).

This sort of alignment is good against running plays because it puts an extra defender “in the box” (close to the line of scrimmage and between the numbers).

When a QB sees this alignment, he knows that the offense is at a disadvantage in the running game and he may change the play into a pass at the line of scrimmage. Offensive coaches often repeat the mantra, “Single high? Let it fly” to drill the concept into the quarterback’s head.   

Out of this alignment, the defense is likely to be running either a Cover 1 man to man defense or a cover 3 zone defense, so the QB will try to call a play that can attack either coverage. We will talk more about how each defense works and how to attack it in a later post, but for now, just learn to recognize single high safety looks and know that they typically tell a QB to expect Cover 1 man or Cover 3 zone.  

MOFO (Middle of Field Open) or Split Safety

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This is called a MOFO defensive shell or a split safety look. It gets its name because the middle of the field is left open due to the dual safeties splitting the field into halves deep in the secondary. This allows the other defenders to be more aggressive in defending the pass, knowing that they have help from their safeties over the top if they get beat deep. While it is typically better against the pass, this alignment is weaker against the run since it has 2 players deep rather than 1. When a QB sees this alignment he may decide to audible from a passing play to a running play to take advantage of this weakness.

Out of this coverage shell, the defense will typically be running Cover 2 man, Cover 2 zone, or Cover 4 zone.

Cover 0

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Though not technically a coverage shell, Cover 0 is the most aggressive defense that exists in football. As you can see in the photo, the circled safeties are not deep in coverage but are instead lined up directly in front of a receiver and closer to the line of scrimmage. It is called Cover 0 because there is no one deep in coverage. The only defense that can be run out of this alignment is Cover 0 man.

The advantage of Cover 0 man is that the defense will rush one more person than the offense has blockers, so the QB will have to get the ball out of his hand quickly. But the downside to the defense is that if a receiver can beat his defender quickly, there is no help over the top to stop the player from scoring a touchdown. It is the ultimate boom or bust defense. A QB won’t see this alignment often, but when he does he knows that he has to check into quick-breaking routes and get the ball out of his hand very fast.

Is It Really That Easy?

I know what you are thinking right now: is it really that easy to read defenses? If it is then anyone could play QB in the NFL. The answer is no, it isn’t really that easy. The defense also knows that the QB will be trying to glean as much information as possible from how they are lined up, so they will sometimes line up wrong and then rotate into the right coverage shell right before the snap or right after the snap happens. Pre-snap reads are only one part of the process. The QB must continue to read the defense after the snap as well while avoiding the rush and finding the open WR.

Each defense is weak at a certain point that the offense will try to attack. And each adjustment the offense makes to exploit the defense will be met with a defensive adjustment to exploit the offense. Every defense has a weakness. Every passing concept has a weakness. He who wins is he who holds the clipboard last. That is what makes football such an exciting and complex game the continues to evolve every season.