Seniority in Congress
In the Senate, the terms Senior Senator and Junior Senator are used to distinguish the two Senators representing a State. Senate Seniority is primarily determined by length of continuous service, and not by age. If two Senators from the same State are sworn in on the same day (a situation requiring at least one special election, since Senators are elected in staggered terms), their relative Seniority is determined by whether one of the two is a former US Senator, US Representative, President, Vice President, Cabinet member or Governor.
Members of Congress are generally given preferential treatment in choosing Committee assignments based on Seniority, with input from party leaders. Seniority on Committees for both Houses of Congress is based on length of time serving on that Committee, and not length of time in Congress. Also, Committee Chairmanships and Ranking Memberships are traditionally given to the most senior member of the majority party serving on the Committee.