These words have common origins in Latin terms meaning "to go before" and "to yield." Precedence refers to "the act or fact of going before," "priority in rank or order," "the right to go in front": "On highways, ambulances have precedence (or take precedence) over civilian cars." "The Secretary of State has precedence over other members of the President's Cabinet." Precedent means a preceding instance, case, or example; in law, it refers to a legal decision that acts as a guide in resolving later cases: "By living thriftily, some parents hope to set a precedent for their children." "The Brown v. Board of Education of Topeka, Kansas, decision of the Supreme Court in 1954 set a precedent for all cases involving public schooling in the United States."
Dictionary of problem words and expressions. Harry Shaw. 1975.