What it is

A single-high shell is a look before the snap with one safety deep in the middle of the field and the other safety closer to the line of scrimmage.

In a game

The deep safety, often called the free safety, protects the middle and helps on deep throws. The second safety plays near the line, where he can help stop the run or cover a receiver. Cover 1 and Cover 3 are played from this look.

More detail

With only one deep safety, the defense has an extra defender near the line, which helps against the run and lets it send an extra pass rusher. The weakness is that one player has to protect the whole deep field, so the cornerbacks must handle deep routes on their own. Offenses read this look before the snap, which is why defenders sometimes disguise it and move just before the ball is snapped.

The diagram

1. One safety stays deep in the middle of the field.
2. The other safety plays near the line of scrimmage.
3. Cover 1 and Cover 3 are played from this shell.