Wing-backs appear in 3-5-2, 3-4-3 and 5-3-2 shapes. They defend as part of a back five without the ball and play as wingers when the team attacks.
The position demands extreme fitness. A wing-back’s sprint count is typically higher than a full-back’s because they cover the entire touchline on their own.
In attack they provide width and crosses; in defence they tuck inside to form a back five. The transition between those two shapes is what makes the system work or fall apart.