Audible

An audible is a play change called by the QB at the line of scrimmage, after he's read the defense. The original call comes from the sideline; the QB confirms or changes it based on the front, the leverage, and the safety rotation he sees.

Modern audibles are usually 'kill' systems — every play has a built-in alternate, and the QB checks to it with a one-word call ('kill, kill') if the original look is bad. Peyton Manning's offenses ran kill systems for two decades. The Chiefs, Bills, and most modern NFL offenses do the same.

The limit on audibles is communication. Crowd noise, time on the play clock, and the offensive line's cadence rules all constrain what the QB can change. Most college offenses have moved past the line-of-scrimmage audible toward the up-tempo signal system, where the play comes in late and the OC effectively audibles for the QB. Either way, the goal is identical — be the last team to commit to a play.