A stole (over both shoulders) is a symbol of his office, and is ordinarily worn when he is celebrating the sacraments.
Sometimes, during a large celebration, a concelebrant may not be fully vested in a chasuble, but he will still wear an alb and a stole. (Which distinguishes him from other ministers who may be about – Deacons, who wear at least an alb and a diagonal stole over one shoulder, or lay ministers, who may be in albs, but no stoles)
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