also: coverage shell · one-high shell · two-high shell
The basic shape of a pass defense, usually described by how many defenders are deep and where they line up. The shell narrows the possible coverages without telling you every assignment.
Coaches use shell for the shape made by the defense's deepest players. Before the snap, a one-high shell shows one deep safety and a two-high shell shows two.
The shell is a starting picture, not the full coverage. A defense can show two high and play Cover 2, quarters or 2-man, or rotate a safety after the snap and finish in a one-high coverage. Cover 4 zone and match quarters can share a four-deep shell while using different rules once the routes develop.
coverage shellSome coaches use shell for the safeties' pre-snap alignment. Others use it for the deep structure of the coverage after the snap. In both cases it describes the outline of the defense, not every player's rule.