Base 4-3 Alignment
For those casually familiar with 4-3 base schemes, this alignment calls for four defensive linemen and three linebackers. The “4” is for these D-linemen to play up along the line of scrimmage, with the three linebackers at the second level.
It’s different from 3-4 alignments, where there are three D-linemen surrounded by two outside linebackers on either edge and two inside linebackers behind the line.
One of the benefits of a 3-4 is it always allows someone to blitz the quarterback. Yet it also requires at least one of the D-lineman to occupy at least two blockers to be moderately successful.
4-3 defenses, matched up against a standard 2WR, 1TE, 2RB base offense, typically look like this:
These defenses are a little less complicated and are often seen as better suited to defend against the run — important, considering San Francisco’s historically bad run defense a year ago.
But the Niners will use a 4-3 under scheme, and even a modification of that with 3-4 personnel.
The 4-3 Under
4-3 under schemes tend to move the SAM — strong-side linebacker — right up on the line of scrimmage. This player is frequently tasked with covering tight ends or getting past him if the tight end is blocking.
The look, as shown below (h/t Sean Nerney), is a direct modification of a Seattle-like defense with this 4-3 under look:
In this look, the 49ers employ five defenders along the line of scrimmage. The thought here is ball carriers will be locked up at the point of attack. And in the pass rush, the defense will be able to overwhelm offensive linemen at the snap.
Cover 3 Hybrid
One of the other elements the 49ers will employ is what’s called a Cover 3 hybrid.
The hybrid is, essentially, a modification of this 4-3 scheme employing a single-high safety. Meanwhile, the strong safety drops down into the box as illustrated in this graphic below:
This is what the Niners will employ quite a bit this season. Defensive back Jimmie Ward will wind up being the 49ers’ single-high safety. Strong safety Eric Reid will be down in the box.
And we’ll break down the specific D-line personnel on subsequent slides.