The nine is football’s most bsic and most important pass route and yet, it is nothing more than a race to the end zone — or at least as far as the Quarterback can throw.
Selling the nine is convincing a defensive back that he is in that race every time a receiver releases from the line of scrimmage.
The nine is basically is a straight line. As such, it’s the stem for many of the other routes a receiver can run. By stem, we refer to another straight line, the one a receiver runs when he escapes the line of scrimmage and races to the breakpoint of his assigned route.
If a receiver can fool a defensive back into thinking he’s going deep, the the underneath routes that work off the nine open up. Separation — the goal of any receiver — becomes easier.
This example of a passing tree is fairly simple, but it shows how the “9” is strictly vertical and other routes break off it. Note how even-numbered routes work to the inside while odd-numbered routes work to the outside.