An informal custom of opening-day proceedings at the beginning of a new Congress is to advise newly sworn freshmen senators of their "number." New members' responses to this information range from confusion to curiosity. "What does my number mean?"
This tradition began in the late 1970s, when a senior senator asked the Senate Historical Office to calculate the number of senators who had served prior to 1959 and to compile a chronological list of all senators who had arrived since that year. This list displayed members' names arranged and numbered by their relative seniority at the time of their initial oath-taking, beginning with Ohio Senator Stephen Young at number 1,572.
By tradition, the Senate determines seniority for the purpose of assigning office space according to former government service and then state population. The highest ranking goes to those with previous service, in descending order, as a senator, vice president, House member, cabinet secretary, and governor. For those with no service in these categories, seniority is calculated according to state population, from largest to smallest.
In 2000 the Historical Office extended this list back to the Senate's first meeting in 1789. Assigning numbers alphabetically within each arriving class, this list designates Richard Bassett of Delaware as senator number one. This custom symbolizes the continuous chain of membership from the present back to the Senate's founding era.Baker, Richard A. The New Members' Guide to Traditions of the United States Senate. (Washington, GPO, 2006. S.Pub. 109-25), 4.