Film From the Field House: How Kyle Shanahan and the 49ers Use the 20 Series

The evolution of professional football from decade to decade is a conversation that never ceases to leave me in awe. Equipment gets stronger and lighter, while athletes eat and train better. The result is a faster game, though football's rudimentary foundations remain unchanged.

San Francisco 49ers head coach Kyle Shanahan's mastery of professional football has given him many labels, including soothsayer and genius.

Predicting the outcome of a play is impressive, but for me, Shanahan's modifications to the original West Coast Offense is his most remarkable accomplishment.

In the 1982 49ers playbook, the late Bill Walsh noted four different drop back passing series: 20s, 50s, 70s, and 80s. The first number indicated the series, and the second was related to which side the Y receiver aligned. For example, '27' would put the tight end on the left or odd side of the formation, while '24' would place the tight end on the right or even side.

The second number also meant different actions for the backs. '24' would be a different flair action for the running backs than '26.'

But the one constant in the 20 series was what both backs did - they split in opposite directions to pick up a linebacker or enter the play on a short route.

Some coaches call the 20 series 'split-flow' protection; Walsh described it as 'divide' or 'split' protection. Don't worry about the terminology; keep your eyes on the running backs and tight end.

Shanahan still uses the 20 series today, although he does not call it often. As previously noted, 'Jet' and 'Scat' protections dominate the 49ers' drop back offense, with scattered use H2/H3 and 64/65 protections.

When the 49ers use 20/21, you'll see six men protecting the quarterback - five offensive linemen and the Y receiver. Typically, 20/21 is used with a single back, and he has a free release.

The center declares the Mike linebacker, setting the protection for the line, which is responsible for the four defensive linemen and the Mike. The Y receiver has responsibility for the Sam, but he can release into the play if the Sam does not blitz, or the offensive tackle takes over the defensive end.

You've probably watched enough football to notice the center declare a Mike backer, and then the quarterback changes the Mike midway through the cadence. The quarterback's ability to 'Re-Mike,' designating a Sam or Will as the Mike linebacker, changes the line's responsibility.

24-25 Chip is all that's left of Walsh's original 20 series. It is seven-man protection, with the offensive line responsible for the four defensive linemen and the Mike. In this series, the Y has a free release, and the backs divide to check outside linebackers.

The strong back reads Sam to the strong safety or corner, while the weak back reads the Will to the free safety or corner.

Week 5: 3rd Quarter - 3rd and 6 at the SF 14 (13:57)

Shanahan called a (3)24/25 protection early in the 3rd quarter against the Cleveland Browns. Running back Raheem Mostert and fullback Kyle Juszczyk made similar reads starting at the outside linebacker and ending with the safety or cornerback.


Both gave an assist, or chip, with their respective tackles. Mostert ended up running into the flat, while Juszczyk stayed in to help pick up the linebacker.

Week 9: 4th Quarter - 3rd and 11 at the SF 16 (8:34)

Shanahan used 25 protection when the 49ers needed a big play on third down against Arizona. As usual, Shanahan put a twist on the call, instructing tight end George Kittle to 'fly' into a gold formation.


Once left tackle Justin Skule had control of the defensive end, Kittle was able to release into the play, while running back Tevin Coleman worked with right tackle Daniel Brunskill to halt defensive end Terrell Suggs.

Later in the game, Shanahan called the same play but flipped it to Gold Right Ace and gave both backs a free release. Quarterback Jimmy Garopplo found wide receiver Emmanuel Sanders on the play for 16 yards.

The 20 series is a dying protection in the modern NFL. There's no point in using two backs to protect a quarterback when a run-pass-option play can freeze a linebacker in his tracks.

However, with the 49ers using two-back sets on nearly 40% of its offensive plays, it makes sense for Shanahan to keep the protection in his playbook.

All images courtesy of NFL.com.
All statistics courtesy of Pro Football Reference unless noted.

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