The shoulder of a tyre is the curved area joining the tread to the sidewall.
A tyre which is under-inflated will wear at the shoulders first. If only one shoulder is worn, it indicates a suspension or steering fault – either caused by wear or less than ideal geometry.
Damage to the shoulder of a tyre cannot usually be repaired.
Shoulder is also a term used in road design, and describes the area next to the carriageway – which is often either paved (hard shoulder) or grassed (soft shoulder).