also: covered · uncovered · covered lineman · uncovered lineman
Whether a defender is aligned on you. It is the first thing a lineman checks before the snap, and in a rule-based scheme it decides his assignment on nearly every play.
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A lineman is covered when a down defender is aligned on him, head up or on either shoulder. In almost every rule-based blocking scheme that is the first branch in his rule. It resolves to the simplest assignment in football: the defender aligned on you is yours, and you block him.
The reason it is the first question rather than a detail is that it is the one thing a lineman can settle on his own, before the snap, without knowing the coverage, the front's name, or what anybody else is doing.
base blockA lineman is uncovered when nobody is aligned on him. He is not free: the scheme sends him somewhere, normally to help the covered lineman next to him on a combo, or straight up to a linebacker.
An uncovered lineman is also the offense's cheapest source of an extra body, which is why defenses care about where their uncovered linemen are. A front that leaves two interior linemen uncovered has handed the offense two players to send at the second level.
climbCovered and uncovered describe ALIGNMENT and not intent. A lineman can be uncovered and still end up blocking a defensive lineman who slanted into him, and that is a scheme problem rather than a mislabel.
This is usually the first blocking rule a young lineman can genuinely execute on his own, because it needs no count, no coverage read and no help from a call.