Strong Safety is the name given to one of the safeties (free safety is the other) to differentiate the two positions and divide responsibilities dependent on scheme.
Most defenses have two safeties, the strong safety and the free safety. Depending on the defensive call and the way the coordinator has set up the defense, each safety will have have a different responsibility. The strong safety is usually the bigger of the two safeties and the better at stopping the run.
If one safety plays closer to the line of scrimmage to try to stop running plays, it is usually the strong safety. The name comes both from the fact that he is to be strong against the run and because he often lines up to the strong side. The strong side is the side of the offense where more blockers are available for a running play.
On coaching diagrams or defensive play cards the safety is often designated as SS or $. When $ is used, S is usually just for the free safety.
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